Add BOARD_CUSTOM_BOOTIMG_MK for custom bootimg building

This patch adds BOARD_CUSTOM_BOOTIMG_MK. It is needed for some devices like
Sony Xperia S (LT26) because it needs a own bootimg format.
If BOARD_CUSTOM_BOOTIMG_MK is not set the build will be done in the normal
way, so other devices are not affected.

Change-Id: I66c8be6b107988d3238001e1dc612c044ed01aaa
Signed-off-by: Andreas Makris <andreas.makris@gmail.com>