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

apps/plugins/crypt_firmware.c

@@ -36,8 +36,7 @@
 */
 
 #include "plugin.h"
-
-
+#include "checksum.h"
 
 static void aes_encrypt(void* data, uint32_t size)
 {
@@ -133,17 +132,6 @@ static void put_uint32be(unsigned char* buf, uint32_t x)
     buf[3] = x & 0xff;
 }
 
-static uint32_t calc_checksum(uint32_t sum, unsigned char* buf, int len)
-{
-   int i;
-
-   for (i = 0; i < len ; i++) {
-      sum += buf[i];
-   }
-
-   return sum;
-}
-
 enum plugin_status plugin_start(const void* parameter)
 {
     int fd;