[rbp/omxplayer] Check total memory, and if plenty use full size buffers

omxplayer has used a quarter sized video buffer and a half sized audio buffer for the file reading message queue, compared to dvdplayer.
This is required on 256M boards, but is unneccessary on 512M board. So we query the memory size and use full size buffers if possible.
Merge pull request #2918 from popcornmix/big_buffer

[rbp/omxplayer] Check total memory, and if plenty use full size buffers