payloads/edk2: Add the declaration for OBJCOPY

The Shimlayer recipe requires OBJCOPY, so declare it at the top of
the Makefile so this recipe works as intended.

Signed-off-by: Sean Rhodes <sean@starlabs.systems>
Change-Id: I2e04dfe18df6252261836dcdf98f7e8de65287b5
Reviewed-on: https://review.coreboot.org/c/coreboot/+/68744
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 ff1dfbc..27ce4a9 100644
--- a/payloads/external/edk2/Makefile
+++ b/payloads/external/edk2/Makefile
@@ -8,6 +8,8 @@
 export EDK2_PATH := $(WORKSPACE)/$(word 3,$(subst /, ,$(CONFIG_EDK2_REPOSITORY)))
 export PACKAGES_PATH := $(EDK2_PATH)
 
+OBJCOPY = $(GCC_PREFIX)objcopy
+
 ifeq ($(CONFIG_EDK2_UEFIPAYLOAD),y)
 BUILD_STR = -p UefiPayloadPkg/UefiPayloadPkg.dsc
 endif