[libc] Fix strcmp()/strncmp() to return proper values

Fix strcmp() and strncmp() to return proper standard positive/negative
values for unequal strings.  Current implementation is backwards
(i.e. the functions are returning negative when should be positive and
vice-versa).

Currently all consumers of these functions only check the return value
for ==0 or !=0 and so we can safely change the implementation without
breaking things.

Signed-off-by: Aaron Young <Aaron.Young@oracle.com>
Modified-by: Michael Brown <mcb30@ipxe.org>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/tests/string_test.c

@@ -88,6 +88,7 @@ static void string_test_exec ( void ) {
 	ok ( strcmp ( "Hello", "hello" ) != 0 );
 	ok ( strcmp ( "Hello", "Hello world!" ) != 0 );
 	ok ( strcmp ( "Hello world!", "Hello" ) != 0 );
+	ok ( strcmp ( "abc", "def" ) < 0 );
 
 	/* Test strncmp() */
 	ok ( strncmp ( "", "", 0 ) == 0 );