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.

TEST=Run 'make' and 'make FIRMWARE_ARCH=arm' successfully

Review URL:

Change-Id: I76738972a8215e346910a76a664a91f6f6927747
diff --git a/firmware/README b/firmware/README
index 2b0bff5..ac35e93 100644
--- a/firmware/README
+++ b/firmware/README
@@ -1,3 +1,5 @@
+arch/ is stuff that defines the architecture-dependent information (only used in
+firmware build mode).
 lib/ is stuff that the BIOS needs to link with.