Revert r333006:

This revision implemented hybrid ISOs for the amd64
architecture, however it was discovered to have caused
a regression in booting legacy-mode (BIOS/CSM).

This restores the way ISOs were previously created, as
the cause (and differences between head and stable/11
and releng/11.2) have not been entirely identified.

Approved by: re (marius)
Sponsored by: The FreeBSD Foundation