envsetup: Set CUSTOM_BUILD environment variable

This is used to gate a few features, including backuptool

Change-Id: I02fee9e0765cca88c833bb0514762aa3b14642e3
Add backuptool support to the releasetools system

Pulled from CyanogenMod 10.1

Change-Id: Iffd9a45865011ad9108508d31f79e53d609dd6f4