payloads/external/Makefile.inc: Make linuxboot a phony target

To quote the gnu make manual: "A phony target is one that is not really
the name of a file; rather it is just a name for a recipe to be executed
when you make an explicit request. There are two reasons to use a phony
target: to avoid a conflict with a file of the same name, and to improve
performance."

Change-Id: I337f4f2e0257a75ba204d21f8aa84292e8233082
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/74309
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Marvin Drees <marvin.drees@9elements.com>
Reviewed-by: Maximilian Brune <maximilian.brune@9elements.com>
Reviewed-by: Lean Sheng Tan <sheng.tan@9elements.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
diff --git a/payloads/external/Makefile.inc b/payloads/external/Makefile.inc
index 475427e..effab43 100644
--- a/payloads/external/Makefile.inc
+++ b/payloads/external/Makefile.inc
@@ -366,7 +366,7 @@
 	MFLAGS= MAKEFLAGS=
 
 # LinuxBoot
-
+.PHONY: linuxboot
 linuxboot:
 	$(MAKE) -C payloads/external/LinuxBoot \
 		CPUS=$(CPUS) \