ginstall: Also check gfactive for active partition

Prowl does not have seperate root filesystem images. This
means ginstall needs a mechanism other than checking rootfs
from the kernel command line. Uboot now adds a new variable
to relay this information with 'gfactive'.

See go/fibercl/79566 for corresponding uboot changes

In reference to b/31805455

Change-Id: Ia8f1670fc6893ed70156671fb7ce81f02a6973a8
5 files changed