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Update loader size on powerpc isos

Summary:
loader has grown to the point it's overflowing the existing 307200 bytes
allocated to it in the HFS boot partition on the isos.

Bump the space reservation up to 500k.

Submitted by: Brandon Bergren
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D20301

Change the way powerpc bootable CDs are generated to work around a bug
in hybrid image generation in cdrtools. This produces a small HFS partition
containing loader, mapped in by an oddly-formed APM table using a new
feature in makefs. This does not appear to work yet on early-model G3
systems, which will be fixed later, but produces bootable CDs on
everything else.