Push some definitions out of preamble.mk into board/

- Require including board/arch.mk
- Move overrides of prebuilts into board/arch.mk to allow per-board
choices
- Move BOOT/ROOTFS_SIZE definitions into board/arch.mk
- Append BOARD_PACKAGES_EXTRA to PACKAGES_EXTRA, to allow per-board
preinstalled packages

Change-Id: I76f00e03036d56bcfd3246c271413b51d2871332
1 file changed