[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/interface/efi/efi_debug.c

@@ -330,7 +330,7 @@ const char * efi_devpath_text ( EFI_DEVICE_PATH_PROTOCOL *path ) {
 		max_len = ( ( sizeof ( text ) - 1 /* NUL */ ) / 2 /* "xx" */ );
 		if ( len > max_len )
 			len = max_len;
-		base16_encode ( start, len, text );
+		base16_encode ( start, len, text, sizeof ( text ) );
 		return text;
 	}