Resize device tree to allow space for board changes and the chosen node

Current code requires that a compiled device tree have space added to the end to
leave room for extra nodes added by board code (and the chosen node).  This
requires that device tree creators anticipate how much space U-Boot will add to
the tree, which is absurd.  Ideally, the code would resize and/or relocate the
tree when it needed more space, but this would require a systemic change to the
fdt code, which is non-trivial.  Instead, we resize the tree inside
boot_relocate_fdt, reserving either the remainder of the bootmap (in the case
where the fdt is inside the bootmap), or adding CFG_FDT_PAD bytes to the size.

Signed-off-by: Andy Fleming <afleming@freescale.com>