base-files: generate /etc/config/wireless, if it doesn't exist

This patch adds a check in "wifi detect" to test if the wireless
configuration file does exist. If it doesn't exist, an empty
/etc/config/wireless file will be created.

This is necessary because uci doesn't create new files,
instead the tool just exits with "uci: Entry not found".

Signed-off-by: Mathias Kresin <dev@kresin.me>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>