hnvram: Added option to choose target file + tests

$HNVRAM_LOCATION envvar can now be used to to read/write from a file
instead of a flash device.

Changed some variable declarations to compile on x86 and allow
end-to-end testing on x86.

Depends on go/fibercl/80569

In reference to b/31822982

Change-Id: I1f41cc9c8351b13d240dbc1fe9d91ceed3bced11
4 files changed