[POWERPC] bootwrapper: Add a firmware-independent simpleboot target.

This target produces a flat binary rather than an ELF file,
fixes the entry point at the beginning of the image, and takes
a complete device tree with no fixups needed.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>