[gdb] Allow resynchronisation with gdb

This replaces the gdbstub's polite NAK behavior with retransmission of
the current outstanding reply packet.  It solves situations where gdb
and gPXE's gdbstub get out of sync due to the lack of flow control in
the gdb protocol spec.

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

src/core/gdbstub.c

 static void gdbstub_state_wait_ack ( struct gdbstub *stub, char ch ) {
 	if ( ch == '+' ) {
 		stub->parse = gdbstub_state_new;
-	} else if ( ch == '-' ) {
-		gdbstub_tx_packet ( stub ); /* retransmit */
-	} else if ( ch == '$' ) {
-		/* GDB is reconnecting, drop our packet and listen to GDB */
-		stub->trans->send ( "-", 1 );
-		stub->parse = gdbstub_state_new;
+	} else {
+		/* This retransmit is very aggressive but necessary to keep
+		 * in sync with GDB. */
+		gdbstub_tx_packet ( stub );
 	}
 }