envsetup: Set CUSTOM_BUILD environment variable

This is used to gate a few features, including backuptool

Change-Id: I02fee9e0765cca88c833bb0514762aa3b14642e3
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
brunch: custom->mintkat.

Change-Id: I5482c765a7c78d6caae371d6fabb8f55e10ac655
build: change custom -> mintkat in a few more places

* these weren't changed when the transtition occured - fix them now

Change-Id: I47bedb96ad63a41959baa98e1a265cb616d17918

Conflicts:
	core/tasks/kernel.mk
	envsetup.sh
Add MintkatROM