payloads/external/Makefile.inc: Fix SeaBIOS option regressions

Commit 14ca7407196 (Makefile.inc: Move adding SeaBIOS cbfs config files)
introduced various regressions that were not spotted during review.

TEST=Building with SEABIOS_THREAD_OPTIONROMS is working properly again.

Change-Id: I4de0b11747e3df8dd31a85160add129d8cc6bd8a
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/47814
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Nico Huber <nico.h@gmx.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
diff --git a/payloads/external/Makefile.inc b/payloads/external/Makefile.inc
index 6208578..0300581 100644
--- a/payloads/external/Makefile.inc
+++ b/payloads/external/Makefile.inc
@@ -121,14 +121,14 @@
 INTERMEDIATE+=seabios_sercon
 seabios_sercon: $(obj)/coreboot.pre $(CBFSTOOL)
 	@printf "    SeaBIOS    Add sercon-port file\n"
-#	$(CBFSTOOL) $@.tmp add-int -i $(CONFIG_SEABIOS_SERCON_PORT_ADDR) -n etc/sercon-port
+	$(CBFSTOOL) $< add-int -i $(CONFIG_SEABIOS_SERCON_PORT_ADDR) -n etc/sercon-port
 endif
 
 ifeq ($(CONFIG_SEABIOS_THREAD_OPTIONROMS),y)
 INTERMEDIATE+=seabios_thread_optionroms
 seabios_thread_optionroms: $(obj)/coreboot.pre $(CBFSTOOL)
 	@printf "    SeaBIOS    Thread optionroms\n"
-	$(CBFSTOOL) $@.tmp add-int -i 2 -n etc/threads
+	$(CBFSTOOL) $< add-int -i 2 -n etc/threads
 endif
 
 # Depthcharge