build/roms: Support using "u-boot" ELF file as U-Boot payload

U-Boot runtime configuration is done with a device-tree file, which is
built alongside the executable in the upstream build system, and must be
available to U-Boot at runtime.

This device-tree is normally not linked into the default "u-boot" ELF
file. So far we have been handling it by re-creating a "u-boot.elf" from
the raw binary parts by setting REMAKE_ELF, and using that as the
coreboot payload. Unfortunately, that fails to build for x86 boards,
more specificly the "coreboot" boards upstream.

It's also possible (but discouraged) to set OF_EMBED to embed the
device-tree file into the U-Boot itself, in which case we could use the
"u-boot" file as the payload on the "coreboot" boards. Add support for
using the "u-boot" file as the payload if "u-boot.elf" doesn't exist.

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>