payloads/external/Linuxboot/Makefile: Guard linux Makefile

The linux.mk makefile needs to be guarded, because it introduces a
dependency on the perl tooling.

Signed-off-by: Maximilian Brune <maximilian.brune@9elements.com>
Change-Id: Ieb7aac672072858e0d6811628887f6a9eb9a8cb1
Reviewed-on: https://review.coreboot.org/c/coreboot/+/76715
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
diff --git a/payloads/external/LinuxBoot/Makefile b/payloads/external/LinuxBoot/Makefile
index 9a9409e..aaa0769 100644
--- a/payloads/external/LinuxBoot/Makefile
+++ b/payloads/external/LinuxBoot/Makefile
@@ -14,7 +14,9 @@
 	ln -s -r $< $@
 
 
+ifeq ($(CONFIG_LINUXBOOT_COMPILE_KERNEL),y)
 include targets/linux.mk
+endif
 ifeq ($(CONFIG_LINUXBOOT_UROOT),y)
 include targets/u-root.mk
 endif