am ed91af6d: am 4bf886ad: (-s ours) am b8c037ba: Merge "Add BOARD_CUSTOM_BOOTIMG_MK for custom bootimg building"

* commit 'ed91af6db5ae8ede196f1e440f3ccae20a4b14cf':
  Add BOARD_CUSTOM_BOOTIMG_MK for custom bootimg building