Only remove $TMP_BUILD_DIR on cleanup

During ramdisk cleanup if for some reason the filesystems mounted
in $TMP_BUILD_DIR were not removed this recursive delete ended up
deleting parts of the host filesystem that were mounted (in particular
most of the contents of /dev got removed.

This commit doesn't deal with the reason the filesystem didn't unmounted
but ensures the consequences are not as severe.

Fixes Bug 1202612

Change-Id: Id8eba0753c2fe76c79ae1d952ce690d26e33e3ed