Merge branch 'cm-11.0' of github.com:CyanogenMod/android_build into cm-11.0

(cherry picked from commit 688076875f5871835febcc089878b71d765e285d)
Store the value from BOARD_CUSTOM_BOOTIMG_MK to a file

This is needed so it can be read in when re-signing a package to
use the proper boot image.

Print notice when using a custom bootimage makefile

Change-Id: I6baab9f8964ce5d4fa9487f2859af9c385d7b21b
(cherry picked from commit 1be6d7051de682abac762bf6ca0c478d8ecbdc37)