partitions of types other than "freebsd-boot" (in particular, "efi").
This allows the removal of some nasty hacks for supporting PowerPC systems,
in particular aliasing freebsd-boot to apple-boot on APM and an IBM-specific
code on MBR.
This changes the installer to use the correct names, which also breaks a
degeneracy in the meaning of "freebsd-boot" that allows the addition
of support for some newer IBM systems that can boot from GPT in addition to
MBR. Since I have no idea how to detect which those systems are, leave
the default on IBM PPC systems as MBR for now.
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