Add "relocatable" flag to images table to indicate that an image can be moved.

Hopefully, my last schema change related to images. If relocatable is not
set then an image must be loaded at the lba_low offset. If set, then the
image can be loaded at other offsets. Currently, all FBSD images are
relocatable courtesy of the relocation mechanism in imagezip (which can
fix up otherwise absolute offsets in an image). Sadly, Linux images are
not relocatable due to absolute block numbers in the grub partition
bootblock that we require. Ryan "taught" imagezip to relocate these, but
I need to find his changes.