rockbox: add a crc32 reverse polynomial function

This uses the reverse of the polynomial used by the current crc_32
function. The code for this was derived from the implementation used
by tinf. This version is space optimized and should be a good way to
reduce code duplication in other parts of rockbox that use the same
crc32 algorithm. This is mainly of use in areas where DEFLATE is in
use.

Change-Id: I918da5b4ea4dc441c0e7e6b5007abcc2da463bcb

apps/plugin.c

@@ -792,6 +792,7 @@ static const struct plugin_api rockbox_api = {
 #ifdef PLUGIN_USE_IRAM
     audio_hard_stop,
 #endif
+    crc_32r,
 
     /* new stuff at the end, sort into place next time
        the API gets incompatible */