unpackfs: code dedup
unpackfs: check for supported filesystems

Use /proc/filesystems to check if the system
supportes the given source filesystem.
locale: make sure /etc/localtime doesn't exist

If /etc/localtime exists in source image, creating a new symlink will fail
and prevent the installer from doing it's job.
Merge pull request #102 from bbqlinux/master

Supported filesystems check in unpackfs, check for /etc/localtime.