Introduce plat_get_skin_dir and update plat_get_root_dir functions

Introduce plat_get_skin_dir and update plat_get_root_dir functions

On GP2X and Pandora, plat_get_root_dir points to the directory
in which the PicoDrive executable is found. On regular Linux,
it now points to the '.picodrive' directory inside the user's home
folder.

plat_get_skin_dir now points to the 'skin' directory inside the
data directory.

The data directory can be set with PICO_DATA_DIR at compile time.
If that variable is not set, the data directory is set to the
directory of the PicoDrive executable.