Redo BOARD_CUSTOM_BOOTIMG_MK support

Simplified version of the previous implementation. Recovery's ramdisk
is spun off from the main recovery target again to allow overriding
just the image-generation step
build: don't clear recovery ramdisk

Clearing recovery ramdisk removes all files place there by the
device tree and all busybox symlinks placed there by recovery
recovery: build uncompressed ramdisk