Improvements for the GCW Zero build

This pull request contains the following changes targeted at the GCW Zero platform (but which might apply to other platforms as well):
1) Better game exit handling. START brings up a pause menu with information about the further actions: START - return to game, SELECT - exit to title. While in the title screen, SELECT exits the program.
2) Improvements to the makefile:
* OSTYPE can be now called from the "make OSTYPE=..." command. If not set, defaults to "msys"
* While called with "make opk", the makefile script creates a GCW Zero compatible OPK file.
3) Added "opk_dir" folder which contains GCW Zero's OPK specific files (contains altered "readme.txt" file with GCW Zero specific information)
4) Iterated game version number to V1.3 (in ./opk_dir/readme.txt only)