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

Conflicts:
	envsetup.sh
cm: Build the BluetoothExt package

 * This package contains Bluetooth profiles implemented by Qualcomm on
   top of Bluedroid.
 * Currently contains support for MAP and FTP profiles.
 * Also included is BTC for wlan coex on QC chipsets.

Change-Id: I933b14de4576691c31087c9de4d60f21cbe3678b