[libc] Rewrite string functions

Some of the C library string functions have an unknown provenance.
Reimplement all such functions to avoid potential licensing
uncertainty.

Remove the inline-assembler versions of strlen(), memswap(), and
strncmp(); these save a minimal amount of space (around 40 bytes in
total) and are not performance-critical.

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

src/hci/strerror.c

@@ -75,7 +75,7 @@ static struct errortab * find_closest_error ( int errno ) {
  * call to strerror().
  *
  */
-const char * strerror ( int errno ) {
+char * strerror ( int errno ) {
 	static char errbuf[64];
 	struct errortab *errortab;