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>

package/base-files/files/bin/board_detect

@@ -8,7 +8,5 @@
 }
 
 [ -f "/etc/board.json" ] || return 1
-[ -f "/etc/config/network" ] || {
-	touch /etc/config/network
-	/bin/config_generate
-}
+
+/bin/config_generate