Refactor armbian-config into install as external package (#7716)

* Refactor armbian-config into install as external package

- https://github.armbian.com/configng repository is always present
- package can still be removed from the install list

Since this tool will have deeper integration with Armbian, we should probably drop possibility to remove the tool from the list.

* Enable armbian-config as extension

extensions/armbian-config.sh

@@ -0,0 +1,12 @@
+# Install armbian config from repo. Now it is producing externally https://github.com/armbian/configng
+# and they are moved to main armbian repo periodically
+
+
+function custom_apt_repo__add_armbian-github-repo(){
+	echo "deb ${SIGNED_BY}https://github.armbian.com/configng stable main" > "${SDCARD}"/etc/apt/sources.list.d/armbian-config.list
+}
+
+
+function post_armbian_repo_customize_image__install_armbian-config() {
+	chroot_sdcard_apt_get_install "armbian-config"
+}