Allocate kernel and ramdisk as low as possible

We currently start the search for free memory for the kernel at
12MB. This made sense back when firmware (real-base) was always
12MB, but it doesn't now it is often at 32MB.

In light of this and now we have fixed the initrd load issues,
just try and allocate the kernel as low as possible.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Tony Breeds <tony@bakeyournoodle.com>