base-files: set pi_ifname in board.d case to fix deconfig

Due to an empty pi_ifname in the generic failsafe setup, the deconfig
never removed the failsafe networking interface, causing broken
networking later on.

Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>

package/base-files/files/lib/preinit/10_indicate_preinit

@@ -90,7 +90,9 @@ preinit_config_board() {
 		ifname=${ifname%\.*}
 	fi
 
-	preinit_ip_config $ifname
+	pi_ifname=$ifname
+
+	preinit_ip_config $pi_ifname
 }
 
 preinit_ip() {