Allow building recovery as boot.

With BOARD_USES_RECOVERY_AS_BOOT = true, we skip building the
non-ramdisk boot.img but building the recovery image as boot.img. It
contains recovery's ramdisk (e.g. with /sbin/recovery). It depends on
the bootloader parameter (skip_initramfs) to determine the actual mode
to boot into.

Change-Id: Id6e2d0a2b94383944ca8f35bba688c6401745622