getconf: Report invalid variables as errors

Printing `undefined` to stdout means that the variable is valid but not
set. Instead, report an error in this case.

linux 4.13 appends the result of `getconf LFS_CFLAGS` to HOSTCFLAGS,
even if it prints `undefined`. This is arguably a bug in the Makefile,
but even so, getconf shouldn't report that unsupported variables are
valid.

getconf.c

@@ -102,7 +102,5 @@ main(int argc, char *argv[])
 		usage();
 	}
 
-	puts("undefined");
-
-	return 1;
+	eprintf("invalid variable: %s\n", argv[0]);
 }