All of the SD card commands are moved into their own file, with
functions sd_init, sd_get_rca, sd_select_card, sd_get_status, and
sd_readblock.
The FAT functions are movied into fat.c and give functions fat_init and
fat_read. Note that the filename is ignored for now, it always reads the
first file in the root directory.
The loading of o65 files is done in o65.c, and executing is done in
exec.c
This cleans up the main file signifigantly and leaves the project open
to expansion.
