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

[mikeioannina]: Squash cm-13.0 changes and adapt to N

Change-Id: I6a744e19612d96a4a6213de0d183366b4730ed0c