Allow setting user name and password from config

Add FIRST_USER_NAME and FIRST_USER_PASS variables that can be set
in the config (or enviroment). Defaults to the standard pi and
raspberry.

stage2/01-sys-tweaks/01-run.sh

@@ -62,12 +62,12 @@ systemctl enable resize2fs_once
 EOF
 fi
 
-on_chroot << \EOF
+on_chroot <<EOF
 for GRP in input spi i2c gpio; do
-	groupadd -f -r "$GRP"
+	groupadd -f -r "\$GRP"
 done
 for GRP in adm dialout cdrom audio users sudo video games plugdev input gpio spi i2c netdev; do
-  adduser pi $GRP
+  adduser $FIRST_USER_NAME \$GRP
 done
 EOF