recovery: fix custom recoveries

This is a squashed set of commits with some updates to minimize conflict with AOSP

recovery: custom recovery build fixes

Patch by Koush

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)

Conflicts:
	core/Makefile

Patch by nemith

remove unused BOARD_USES_COMBINED_RECOVERY

Change-Id: I79520c510488d63d469b0d1a3a099420f7de1fe6

Patch by codeworkx

Change-Id: I6bcab88b76272526bdc558fbf8d7b43347bb5ef1