[base16] Add buffer size parameter to base16_encode() and base16_decode()

The current API for Base16 (and Base64) encoding requires the caller
to always provide sufficient buffer space.  This prevents the use of
the generic encoding/decoding functionality in some situations, such
as in formatting the hex setting types.

Implement a generic hex_encode() (based on the existing
format_hex_setting()), implement base16_encode() and base16_decode()
in terms of the more generic hex_encode() and hex_decode(), and update
all callers to provide the additional buffer length parameter.

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

src/drivers/net/ecm.c

@@ -105,7 +105,7 @@ int ecm_fetch_mac ( struct usb_device *usb,
 		return -EINVAL;
 
 	/* Decode MAC address */
-	len = base16_decode ( buf, hw_addr );
+	len = base16_decode ( buf, hw_addr, ETH_ALEN );
 	if ( len < 0 ) {
 		rc = len;
 		return rc;