[new uImage] Use lmb for bootm allocations

Convert generic ramdisk_high(), get_boot_cmdline(), get_boot_kbd()
functions over to using lmb for allocation of the ramdisk, command line
and kernel bd info.

Convert PPC specific fdt_relocate() to use lmb for allocation of the device
tree.

Provided a weak function that board code can call to do additional
lmb reserves if needed.

Also introduce the concept of bootmap_base to specify the offset in
physical memory that the bootmap is located at.  This is used for
allocations of the cmdline, kernel bd, and device tree as they should
be contained within bootmap_base and bootmap_base + CFG_BOOTMAPSZ.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>