Environment variables

KCPPFLAGS
--------------------------------------------------
Additional options to pass when preprocessing. The preprocessing options
will be used in all cases where kbuild does preprocessing including
building C files and assembler files.

KAFLAGS
--------------------------------------------------
Additional options to the assembler.

KCFLAGS
--------------------------------------------------
Additional options to the C compiler.

KBUILD_VERBOSE
--------------------------------------------------
Set the kbuild verbosity. Can be assigned same values as "V=...".
See make help for the full list.
Setting "V=..." takes precedence over KBUILD_VERBOSE.

KBUILD_EXTMOD
--------------------------------------------------
Set the directory to look for the kernel source when building external
modules.
The directory can be specified in several ways:
1) Use "M=..." on the command line
2) Environmnet variable KBUILD_EXTMOD
3) Environmnet variable SUBDIRS
The possibilities are listed in the order they take precedence.
Using "M=..." will always override the others.

KBUILD_OUTPUT
--------------------------------------------------
Specify the output directory when building the kernel.
The output directory can also be specificed using "O=...".
Setting "O=..." takes precedence over KBUILD_OUTPUT.

ARCH
--------------------------------------------------
Set ARCH to the architecture to be built.
In most cases the name of the architecture is the same as the
directory name found in the arch/ directory.
But some architectures such as x86 and sparc have aliases.
x86: i386 for 32 bit, x86_64 for 64 bit
sparc: sparc for 32 bit, sparc64 for 64 bit

CROSS_COMPILE
--------------------------------------------------
Specify an optional fixed part of the binutils filename.
CROSS_COMPILE can be a part of the filename or the full path.

CROSS_COMPILE is also used for ccache is some setups.

CF
--------------------------------------------------
Additional options for sparse.
CF is often used on the command-line like this:

    make CF=-Wbitwise C=2

INSTALL_PATH
--------------------------------------------------
INSTALL_PATH specifies where to place the updated kernel and system map
images. Default is /boot, but you can set it to other values.

INSTALLKERNEL
--------------------------------------------------
Install script called when using "make install".
The default name is "installkernel".

The script will be called with the following arguments:
    $1 - kernel version
    $2 - kernel image file
    $3 - kernel map file
    $4 - default install path (use root directory if blank)

The implmentation of "make install" is architecture specific
and it may differ from the above.

INSTALLKERNEL is provided to enable the possibility to
specify a custom installer when cross compiling a kernel.

MODLIB
--------------------------------------------------
Specify where to install modules.
The default value is:

     $(INSTALL_MOD_PATH)/lib/modules/$(KERNELRELEASE)

The value can be overridden in which case the default value is ignored.

INSTALL_MOD_PATH
--------------------------------------------------
INSTALL_MOD_PATH specifies a prefix to MODLIB for module directory
relocations required by build roots.  This is not defined in the
makefile but the argument can be passed to make if needed.

INSTALL_MOD_STRIP
--------------------------------------------------
INSTALL_MOD_STRIP, if defined, will cause modules to be
stripped after they are installed.  If INSTALL_MOD_STRIP is '1', then
the default option --strip-debug will be used.  Otherwise,
INSTALL_MOD_STRIP will used as the options to the strip command.

INSTALL_FW_PATH
--------------------------------------------------
INSTALL_FW_PATH specifies where to install the firmware blobs.
The default value is:

    $(INSTALL_MOD_PATH)/lib/firmware

The value can be overridden in which case the default value is ignored.

INSTALL_HDR_PATH
--------------------------------------------------
INSTALL_HDR_PATH specifies where to install user space headers when
executing "make headers_*".
The default value is:

    $(objtree)/usr

$(objtree) is the directory where output files are saved.
The output directory is often set using "O=..." on the commandline.

The value can be overridden in which case the default value is ignored.

KBUILD_MODPOST_WARN
--------------------------------------------------
KBUILD_MODPOST_WARN can be set to avoid errors in case of undefined
symbols in the final module linking stage. It changes such errors
into warnings.

KBUILD_MODPOST_NOFINAL
--------------------------------------------------
KBUILD_MODPOST_NOFINAL can be set to skip the final link of modules.
This is solely useful to speed up test compiles.

KBUILD_EXTRA_SYMBOLS
--------------------------------------------------
For modules that use symbols from other modules.
See more details in modules.txt.

ALLSOURCE_ARCHS
--------------------------------------------------
For tags/TAGS/cscope targets, you can specify more than one arch
to be included in the databases, separated by blank space. E.g.:

    $ make ALLSOURCE_ARCHS="x86 mips arm" tags