Simplify MBR and install it as /usr/mdec/mbr instead of /usr/mdec/mbr.mini
since it makes a fairly decent template for fdisk -i. Change the procedure
that makes the rb600 installation miniroot image to use fdisk -i instead of
dd(1) to create the MBR for the image. Adjust the miniroot disk type in
/etc/disktab to the slightly different layout that fdisk -i creates.