Allow numbers in username

packages/bsp/common/usr/lib/armbian/armbian-firstlogin

@@ -249,7 +249,7 @@ add_user()
 	while [ -f "/root/.not_logged_in_yet" ]; do
 		echo -e "\nPlease provide a username (eg. your first name): \c"
 		read -r -e username
-		if ! grep '^[a-zA-Z]*$' <<< "$username" > /dev/null ; then
+		if ! grep '^[a-zA-Z0-9]*$' <<< "$username" > /dev/null ; then
 			echo -e "\n\x1B[91mError\x1B[0m: illegal characters in username"
 			return
 		fi