releasetools: use prebuilt boot image if using a custom boot image makefile

Change-Id: Ic348d31e8df7933dc4f9f055b7a5a2beacc21d32
Bionic optimization for Cortex A5

Change-Id: I06f29035fcfa2849b2d6f46dbfc7273312b1d136
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
  CM_FIXUP_COMMON_OUT is set.
* Refer https://groups.google.com/forum/#!topic/android-building/ispbOgzoyg8
  for more info.

Change-Id: I11e7df0e68e2a60ce32576f06397d60fc9465b60