Give the user the option to simply register the SPI Flash MTD as a
device node on /dev.
Currently, this is achievable only when SmartFS (which is the default FS
option) is disabled by force. This behavior is fixed by using the
"optional" Kconfig keyword, which makes the "choice" selectable.
Signed-off-by: Gustavo Henrique Nihei <gustavo.nihei@espressif.com>