Add a unique build number to every build

SDK-readable via android.os.Build.INCREMENTAL

Change-Id: If9885adebe12a56b2b2e353285721be695bc411b
Fix exporting the unique build identifier

Change-Id: Ie58e981b7e83a4351d137dfd8995dce7d365d950
Merge branch 'cm-10.2' of http://github.com/CyanogenMod/android_build into cm-10.2