[efi] Verify object format support in elf2efi.c

Currently, if you attempt to build 64-bit EFI binaries on a 32-bit
system without a suitable cross-compiling version of libbfd, the iPXE
build will die with a segmentation fault in elf2efi64.

Fix by properly handling the return value from bfd_check_format().

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/util/elf2efi.c

 	/* The call to bfd_check_format() must be present, otherwise
 	 * we get a segfault from later BFD calls.
 	 */
-	if ( bfd_check_format ( bfd, bfd_object ) < 0 ) {
-		eprintf ( "%s is not an object file\n", filename );
+	if ( ! bfd_check_format ( bfd, bfd_object ) ) {
+		eprintf ( "%s is not an object file: ", filename );
+		bfd_perror ( NULL );
 		exit ( 1 );
 	}