#29 Add information of availability of EFS

[Reylak]

We could add either a function or a variable inside System to tell if EFS is present. Probably something like System.EFS or System.isEFSAvailable(), which is true if we have an embedded file system.

On how to do this, I can imagine setting a macro on compilation (which is where we determine if an EFS will be appended to the .nds) using gcc's option -D. It would set ULUA_EFS so then when compiling main.c we have something that test the definition of the macro and push the right value into System.EFS accordingly.

I know we can test if 'efs:' can be accessed through System.changeCurrentDirectory() but a clean way is better.

[Reylak]

Done in [feaaf5] "Fix the too long startup."

This commit adds the variable System.EFS, which is a boolean being true if there is an Embedded File System, and false otherwise.