[rbp] Enable optimization on Raspberry Pi

Originally disabled by commit 177bc0b to work around FFmpeg issue. I believe that is no longer relevant since FFmpeg optimization flags are handled separately now. Building core Kodi code with -O2 does not produce any ill effects on Raspberry Pi and should be a good thing to enable by default since performance is critical on that platform. Tested on Raspbian Jessie with GCC 4.9.1.