base-files: rework config generation logic

Now that config_generate is able to generate the entire /etc/config/system
from scratch we can apply the same logic as used for /etc/config/network;
when the configuration file exists already then do not do anything, else
generate it from the values provided by /etc/board.json .

In order to facilitate that move the file existance checking inside
/bin/config_generate and call it unconditionally from /bin/board_detect.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>