configs: EXT4, FAT, hush shell, env on S-mode MAIX

* enable storing the environment in the SPI flash
* enable EXT4 and FAT file system
* enable hush shell
* run k210_bootcmd as default boot command

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Sean Anderson <seanga2@gmail.com>