payloads/edk2: Guard the build target

Specifying a build target only applies to UefiPayloadPkg, so guard it
against the relevant Kconfig option.

Signed-off-by: Sean Rhodes <sean@starlabs.systems>
Change-Id: Ia4597b5ed76616e39cec45f8a69be9f1ccd72d4c
Reviewed-on: https://review.coreboot.org/c/coreboot/+/68182
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Lean Sheng Tan <sheng.tan@9elements.com>
diff --git a/payloads/external/edk2/Makefile b/payloads/external/edk2/Makefile
index f9a48fb..041d282 100644
--- a/payloads/external/edk2/Makefile
+++ b/payloads/external/edk2/Makefile
@@ -8,7 +8,9 @@
 export EDK2_PATH := $(WORKSPACE)/$(word 3,$(subst /, ,$(CONFIG_EDK2_REPOSITORY)))
 export PACKAGES_PATH := $(EDK2_PATH)
 
+ifeq ($(CONFIG_EDK2_UEFIPAYLOAD),y)
 BUILD_STR = -p UefiPayloadPkg/UefiPayloadPkg.dsc
+endif
 BUILD_STR += -t COREBOOT
 BUILD_STR += -D BOOTLOADER=COREBOOT
 ifneq ($(CONFIG_EDK2_VERBOSE_BUILD),y)