Makefile: add rule to generate u-boot-swap.bin

This rule generates an u-boot binary file where the byte endianness
is swapped. This will be used by the MIPS Malta Little-Endian variants
to be able to boot with Qemu. The Qemu Malta Machine expects the
firmware in Big-Endian order.

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>