Unbreak and clean up PReP (PowerPC Reference Platform) system emulation.
The 'prep' machine was removed in QEMU v5.0. The '40p' machine should be used now instead.
Old work was done at http://repo.or.cz/w/qemu/afaerber.git/shortlog/refs/heads/prep
Some assertions were fixed. Device emulation was improved. Kernels are still not booting.
Old patch queue: http://repo.or.cz/w/qemu/afaerber.git/shortlog/refs/heads/prep-queue