falter-berlin-admin-keys: fix $OPKG_INSTROOT for buildroot builds

Author: pktpls

packages/falter-berlin-admin-keys/files/register_keys.sh

@@ -7,7 +7,7 @@ KEY_FILES=$(find "$SCRIPTPATH" -name "*.pub")
 
 for KEY_FILE in $KEY_FILES; do
     KEY=$(cat $KEY_FILE)
-    if [ -z "$(grep "$KEY" /etc/dropbear/authorized_keys)" ]; then
-        echo "$KEY" >> /etc/dropbear/authorized_keys
+    if [ -z "$(grep "$KEY" $IPKG_INSTROOT/etc/dropbear/authorized_keys)" ]; then
+        echo "$KEY" >> $IPKG_INSTROOT/etc/dropbear/authorized_keys
     fi
 done