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.

stage1/01-sys-tweaks/00-run.sh

@@ -5,10 +5,10 @@ install -m 644 files/noclear.conf "${ROOTFS_DIR}/etc/systemd/system/getty@tty1.s
 install -v -m 644 files/fstab "${ROOTFS_DIR}/etc/fstab"
 
 on_chroot << EOF
-if ! id -u pi >/dev/null 2>&1; then
-	adduser --disabled-password --gecos "" pi
+if ! id -u ${FIRST_USER_NAME} >/dev/null 2>&1; then
+	adduser --disabled-password --gecos "" ${FIRST_USER_NAME}
 fi
-echo "pi:raspberry" | chpasswd
+echo "${FIRST_USER_NAME}:${FIRST_USER_PASS}" | chpasswd
 echo "root:root" | chpasswd
 EOF