Fix false positive when checking ELF prebuilts in PRODUCT_COPY_FILES

llvm-readobj is valid for other object file types such as PE/COFF, so
some firmware files are wrongly detected as ELF prebuilts. Additionally
check the output of llvm-readobj to avoid such false positives.

Change-Id: I4c4ede747967b4a1213aadaed2f925672d18579c

core/definitions.mk

@@ -3206,7 +3206,7 @@ $(check_non_elf_file_timestamp): $(1) $(LLVM_READOBJ)
 	$(hide) mkdir -p "$$(dir $$@)"
 	$(hide) rm -f "$$@"
 	$(hide) \
-	    if $(LLVM_READOBJ) -h "$$<" >/dev/null 2>&1; then \
+	    if $(LLVM_READOBJ) -h "$$<" 2>/dev/null | grep -q "^Format: elf"; then \
 	        $(call echo-error,$(2),$(3)); \
 	        $(call echo-error,$(2),found ELF file: $$<); \
 	        false; \