Fix it to work under FreeBSD

Add sp_drain() where needed. Without it FreeBSD won't pass the handshake
phase.
Also decreate max buffer size.
Tested with Pinecil v2 and it can write and check new firmware.

lib/blisp.c

@@ -13,6 +13,12 @@
 
 #define DEBUG
 
+static void drain(struct sp_port* port) {
+#if defined(__APPLE__) || defined(__FreeBSD__)
+    sp_drain(port);
+#endif
+}
+
 int32_t blisp_device_init(struct blisp_device* device,
                           struct blisp_chip* chip) {
   device->chip = chip;
@@ -130,9 +136,7 @@ int32_t blisp_send_command(struct blisp_device* device,
     blisp_dlog("Received error or not written all data: %d", ret);
     return BLISP_ERR_UNKNOWN;
   }
-#ifdef __APPLE__
-  sp_drain(serial_port);
-#endif
+  drain(serial_port);
 
   return BLISP_OK;
 }
@@ -195,12 +199,12 @@ int32_t blisp_device_handshake(struct blisp_device* device, bool in_ef_loader) {
     if (!in_ef_loader) {
       if (device->is_usb) {
         sp_blocking_write(serial_port, "BOUFFALOLAB5555RESET\0\0", 22, 100);
-#ifdef __APPLE__
-        sp_drain(serial_port);
-#endif
+        drain(serial_port);
       }
     }
     ret = sp_blocking_write(serial_port, handshake_buffer, bytes_count, 500);
+	// not sure about Apple part, but FreeBSD needs it
+	drain(serial_port);
     if (ret < 0) {
       blisp_dlog("Handshake write failed, ret %d", ret);
       return BLISP_ERR_UNKNOWN;