envsetup: More PROMPT_COMMAND

* Get rid of ANDROID_NO_PROMPT_COMMAND, the pre-existing STAY_OFF_MY_LAWN
does the same thing
* Remove any pre-existing android prefixes to avoid accumulation
scenarios in consecutive builds (i.e., multiple build identification
strings in hardstatus)

Change-Id: I86661e066e18ea6ad0c742fa1dc6555780fa5441
Merge branch 'cm-10.1' of github.com:CyanogenMod/android_build into cm-10.1