PPC: calculate kernel,initrd,cmdline locations dynamically

PPC: calculate kernel,initrd,cmdline locations dynamically

During testing, I was generating a vmlinux binary that easily occupied
more than 20MB of RAM. Since the current -kernel code loads the initrd
at a fixed address behind the kernel, we were overwriting kernel data
when the kernel got too big.

To finally get rid of the issue, let's calculate the initrd and cmdline
addresses relative to the kernel size, so we can have kernels and initrds
that are as big as they want to - as long as they fit in RAM.

Signed-off-by: Alexander Graf <agraf@suse.de>