[POWERPC] Split low-level OF-related bootloader code into separate files

Currently, all OF-related code in the bootloader is contained in of.c.
of.c also provides the platform specific things necessary to boot on
an OF platform.

However, there are platforms (such as PReP) which can include an OF
implementation, but are not bootable as pure OF systems.  For use by
such platforms, this patch splits out the low-level parts of the OF
code (call_prom() and various wrappers thereof) into a new oflib.c
file.  In addition, the code related to bootwrapper console output via
OF are moved to a new ofconsole.c file.  Both these files are included
in the wrapper.a library where they can be used by both full-OF and
partial OF platforms.

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