rootgen.sh: complete all profiles except GELI+MBR (not supported)

This extends the test suite to generate images for every combination of:
amd64: mbr/gpt geli/nogeli ufs/zfs legacy/uefi/both

Except for mbr+geli, which is not currently possible.

Reviewed by: imp (previous version)
Sponsored by: Klara Systems
Differential Revision: https://reviews.freebsd.org/D15846