build/roms: copy base rom again for u-boot

when building only for u-boot, the current script
works just fine. however, when building for other
payloads in additional to u-boot, the final u-boot
stage fails because other payloads are already
inserted via cbfs.

when we build u-boot, we do that last because we want
u-boot setups to only be u-boot, nothing else.

this patch enables qemu x86 to build properly with
u-boot.

Signed-off-by: Leah Rowe <leah@libreboot.org>

script/build/roms

@@ -232,6 +232,7 @@ build_roms()
 	[ "${payload_grub}" != "y" ] || \
 		x_ build_grub_roms "${cbrom}" "grub"
 	[ "${payload_uboot}" = "y" ] || return 0
+	x_ cp "${_cbrom}" "${cbrom}"
 	build_uboot_roms
 }