Intel has invented yet another binary blob which firmware is required to
run. This is run after SDRAM is ready. It is linked to load at a particular
address, typically 0, but is a relocatable ELF so can be moved if required.

Add support for this in the build system. The file should be placed in the
board directory, and called refcode.elf.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>