[PATCH] powerpc: Add back support for booting from BootX (#2)

ARCH=powerpc couldn't boot from BootX as it uses a "different" way of
getting in the kernel. This patch adds the necessary trampolines,
creating a flattened device-tree from the tree passed from MacOS, and
initializing the btext engine early for really-early debugging.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>