rockbox: move firmware checksum algorithms into a common header

Before this was just implemented inline wherever it was needed. Now
it is provided as 2 inline functions in a header called checksum.h.
There should be no differences in actual behavior.

Change-Id: I5d756cc01dc6225f5cc8b6af90911a3fc7b57cd5

firmware/include/checksum.h

@@ -0,0 +1,41 @@
+/***************************************************************************
+ *             __________               __   ___.
+ *   Open      \______   \ ____   ____ |  | _\_ |__   _______  ___
+ *   Source     |       _//  _ \_/ ___\|  |/ /| __ \ /  _ \  \/  /
+ *   Jukebox    |    |   (  <_> )  \___|    < | \_\ (  <_> > <  <
+ *   Firmware   |____|_  /\____/ \___  >__|_ \|___  /\____/__/\_ \
+ *                     \/            \/     \/    \/            \/
+ * $Id$
+ *
+ * Copyright (C) 2020 James Buren
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
+ * KIND, either express or implied.
+ *
+ ****************************************************************************/
+#include <stdint.h>
+#include <stdbool.h>
+
+#ifndef _CHECKSUM_H
+#define _CHECKSUM_H
+
+/* rockbox firmware checksum algorithm */
+static inline uint32_t calc_checksum(uint32_t sum, const uint8_t* buf, size_t len)
+{
+    for (size_t i = 0; i < len; i++)
+        sum += buf[i];
+    return sum;
+}
+
+/* similar to above but only used for verification */
+static inline bool verify_checksum(uint32_t cs, const uint8_t* buf, size_t len)
+{
+    return (calc_checksum(MODEL_NUMBER, buf, len) == cs);
+}
+
+#endif