ppc / sparc: Add a tester for checking whether OpenBIOS runs successfully

Since the mac99 and g3beige PowerPC machines recently broke without
being noticed, it would be good to have a tester for "make check"
that detects such issues immediately. A simple way to test the firmware
of these machines is to use the "-prom-env" parameter of QEMU. This
parameter can be used to put some Forth code into the 'boot-command'
firmware variable which then can signal success to the tester by
writing a magic value to a known memory location. And since some of the
Sparc machines are also using OpenBIOS, they are now tested with this
prom-env-tester, too.

Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
[dwg: Removed sparc64, because it trips a TCG bug on 32-bit hosts]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>