Add HAMMER support to the installer

Add HAMMER support to the installer

Commit the installer work done at the 25c3 in Berlin.  This adds experimental
HAMMER support to the installer.  Note: the current version requires a recent
master which is able to boot from HAMMER.  I dropped the UFS /boot support
in favour for HAMMER boot support (yes, I know about the issues, but I'm sure
you guys will get them solved :)

Note: This is a first version, so do not blame me if the installer eats up
your hard disk :)