cm.mk: don't override BUILD_ID

Change-Id: I3a0040ee65cefb25908c9710b324a319a468d5b6
Merge "cm.mk: don't override BUILD_ID" into cm-11.0
Merge branch 'cm-11.0' of https://github.com/CyanogenMod/android_device_htc_dlx into cd-4.4