boot/syslinux: define OBJCOPY and AS

This fixes build on an ARM64 host (and I assume also
other non-x86).

Signed-off-by: Urja Rannikko <urjaman@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
(cherry picked from commit e938419bd98d57428aac9fc881c7a4adf45a5316)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

boot/syslinux/syslinux.mk

@@ -60,6 +60,8 @@ define SYSLINUX_BUILD_CMDS
 	$(TARGET_MAKE_ENV) $(MAKE1) \
 		CC="$(TARGET_CC)" \
 		LD="$(TARGET_LD)" \
+		OBJCOPY="$(TARGET_OBJCOPY)" \
+		AS="$(TARGET_AS)" \
 		NASM="$(HOST_DIR)/bin/nasm" \
 		CC_FOR_BUILD="$(HOSTCC)" \
 		CFLAGS_FOR_BUILD="$(HOST_CFLAGS)" \