[POWERPC] Consolidate cuboot initialization code

The various cuboot platforms (i.e. pre-device tree aware u-boot for
83xx, 85xx and Ebony) share a certain amount of code for parsing the
boot parameters.  To a certain extent that's inevitable, since they
platforms have different definitions of the bd_t structure.  However,
with some macro work and a helper function, this patch improves the
situation a bit.

In the process, this fixes a bug on Ebony, which was incorrectly
handling the parameters passed form u-boot for the command line (the
bug was copied from 83xx and 85xx which have subsequently been fixed).

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>