envsetup: add a helper to fix "out/target/common" not being really common

* out/target/common isn't really common, and if you do
  back to back builds for multiple devices, that is noticable.
* Use out/target/common-$device instead, and link the appropriate
  dir to out/target/common every time lunch() is run, if
  ANDROID_FIXUP_COMMON_OUT is set.
* Refer https://groups.google.com/forum/#!topic/android-building/ispbOgzoyg8
  for more info.

Change-Id: I11e7df0e68e2a60ce32576f06397d60fc9465b60

Conflicts:
	envsetup.sh
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

Conflicts:
	core/Makefile