MIPS: make sure to fill 'jr' branch delay slot with 'nop'

Inline assembly in RoLO and the FiiO M3K bootloader used 'jr' to
jump to a newly loaded Rockbox binary, but incorrectly left the
branch delay slot open. That gives GCC an opening to place illegal
instrutions, etc, which might cause an unhandled exception.

Change-Id: Ia7a561fe530e94a41189d25f18a767c448177960

firmware/rolo.c

@@ -204,6 +204,7 @@ void rolo_restart(const unsigned char* source, unsigned char* dest,
     commit_discard_idcache();
     asm volatile(
         "jr     %0               \n"
+        "nop\n"
         : : "r"(dest)
     );
 #endif