Simplify ROM filename, s/SNAPSHOT/EXPERIMENTAL/, other cleanups

* Remove "update-" prefix and "-signed" suffix.
* Replace SNAPSHOT with EXPERIMENTAL (arcee).
* If CM_EXTRAVERSION, force to type EXPERIMENTAL.
* UNOFFICIAL builds now with datestamp in the same manner as NIGHTLY.
* date -u uses UTC timezone for consistency.
* Remove some CM7 dead code.

Change-Id: I681fd92e27aaf62c50eae20265ce07c67d477bca