Add firmware build mode to Makefiles

This CL also includes a biosincludes.h for ARM platform.

The changes to ebuilds are in a separated CL:5352002.

BUG=None
TEST=Run 'make' and 'make FIRMWARE_ARCH=arm' successfully

Review URL: http://codereview.chromium.org/5301004

Change-Id: I76738972a8215e346910a76a664a91f6f6927747
4 files changed