Clean up arch.mk, only define really board-specific things here

- Define ROOTFS_RAW_CACHE_DIRECTORY
- Define an empty (for now) BOARD_PACKAGE_EXTRA, which is appended to
PACKAGES_EXTRA in build/preamble.mk
- Fix up ROOTFS_SIZE_MB

Change-Id: Icb2647998feab529d615025109bccecbbff97194
1 file changed