Add BOARD_USES_INITRAMFS and BOARD_USES_COMBINED_RECOVERY.

BOARD_USES_INITRAMFS makes recovery.img and boot.img bare cpio
archives, so that they can be used directly when building a kernel
for targets that only support embedded initramfs, and ensures that
dependencies from kernel to boot/recovery images are broken.
BOARD_USES_COMBINED_RECOVERY produces a combined recovery that can be
used for boot or recovery.

Change-Id: Idbd07009a3f84ab27b5a3d35e286834d8d32d3d4