medkit-initial-config: add new package

The idea of this package is to allow users that do not have ethernet port to configure router anyway. Usage is to place configure file alongside the medkit and do medkit reflash. This wipes and updates current installation (hopefully to latest version as user should always download latest one) and on first boot (detected using schnapps as there are no snapshots in such case) script from this package locates config the same way as in rescue is medkit detected. Configuration is read and applied to system. That is even before web server or even network interface is started so it makes router much secure on first boot.

updater/initial-config/files/README.md

+Initial system configuration
+----------------------------
+This package provides script that allows limited configuration of router after
+factory reset or medkit reflash is used. The idea is to allow users to
+preconfigure router in a way they can connect to it securely over Wi-Fi if
+needed.
+
+## Usage
+User places configuration file to root directory of a flash drive. The name of
+the file has to be: `medkit-config.json`. Such prepared flash drive is inserted to
+router and factory reset or medkit reflash is performed. Flash drive has to be
+kept in device till you access we interface.
+
+Make sure to wipe configuration file from flash drive after that to not disclose
+accidentally router's password when you reuse it. Other option is to change both
+Foris and system password after initial setup.
+
+## Configuration file format
+Configuration file has to contain valid JSON.
+
+### Example configuration
+```
+{
+	"foris_password": "ForisPassword_ChangeThis!",
+	"system_password": "SystemPassword_ChangeThis!",
+	"wireless": {
+		"ssid": "TurrisConfigWifi",
+		"key": "WiFiPassword_ChangeThis!"
+	}
+}
+```
+
+### Foris Password
+Option `foris_password` can be used to configure password for Foris web interface
+and that way skip initial step in setup.
+
+This is required to be used as web interfaces allows anyone to set initial
+password. That makes router administration accessible by anyone. By setting
+password even before Wi-Fi or/and Foris are started prevents access to just
+everyone.
+
+### System Password
+Option `system_password` can be used to configure password for `root` account on
+router. This is password used by LuCI web interfaces as well as SSH.
+
+This is not essentially required on Turris, because in default root account is
+blocked for interactive login. This is included rather for convenience for cases
+when user wants to use SSH rather than Foris.
+
+### Wireless AP configuration
+Option `wireless` has to be set to object with `ssid` and `key` fields. It
+configures first radio it can access on system to AP mode with provided SSID and
+key (password).