[POWERPC] zImage: Cleanup and improve prep_kernel()

This patch rewrites prep_kernel() in the zImage wrapper code to be
clearer and more flexible.  Notable changes:

	- Handling of the initrd image from prep_kernel() has moved
into a new prep_initrd() function.
	- The address of the initrd image is now added as device tree
properties, as the kernel expects.
	- We only copy a packaged initrd image to a new location if it
is in danger of being clobbered when the kernel moves to its final
location, instead of always.
	- By default we decompress the kernel directly to address 0,
instead of requiring it to relocate itself.  Platforms (such as OF)
where doing this could clobber still-live firmware data structures can
override the vmlinux_alloc hook to provide an alternate place to
decompress the kernel.
	- We no longer pass lots of information between functions in
global variables.

Signed-off-by: David Gibson <dwg@au1.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>