build/fw/coreboot: Fix misuse of raw u-boot.bin as payload

The "u-boot.bin" file generated by U-Boot builds is a raw binary. When
adding payloads to a CBFS, we need to use ELF files with add-payload
or manually pass the entry point and load address of the payload binary
with add-flat-binary.

We primarily use the "u-boot.elf" which gets build with the REMAKE_ELF
option, as it also has the necessary device-tree binary that U-Boot
usually needs to work. When the option is not set (e.g. for QEMU), we
need to use the "u-boot" file which is an ELF.

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

config/u-boot/build.list

@@ -1,3 +1,4 @@
+u-boot
 u-boot.bin
 u-boot.dtb
 u-boot.img