Extract uboot image creation from core/Makefile

http://memegenerator.net/instance/12495768

This change utilizes BOARD_CUSTOM_BOOTIMG_MK to extract out the Uboot boot and
recovery image generation out into it's own file (approx 130 lines).

This should:
    * Clean up the already huge Makefile and the keep ifdeffyness down
    * Make sure Uboot changes don't effect other builds
    * Extract a lot of code that is only for 3 devices

This depreciates BOARD_USES_UBOOT but BOARD_USES_UBOOT_MULTIIMAGE is still valid.

Boards who use uboot boot images should remove BOARD_USES_UBOOT and add
  BOARD_CUSTOM_BOOTIMG_MK := device/common/uboot-bootimg.mk
to their BoardConfig.mk
(location of the makefile is not finalized)

Change-Id: I90505a99cbcda11731ac3318cfe7f76b4bf6e1c7