Fetch only external repos for repo mirror

Change-Id: I55a8aa3d69a3df5337a8a61268bde91adb091ab0
Reviewed-by: Teemu Holappa <teemu.holappa@theqtcompany.com>

b2qt-init-build-env

@@ -165,7 +165,7 @@ mirror() {
   mkdir -p .repo/manifests
   cp ${DIR}/scripts/manifest.xml .repo/manifests/
   MANIFEST="manifest.xml"
-  ${REPO} init ${REPO_URL} -u ${PWD}/.repo/repo -b default -m ${MANIFEST} -g all --mirror
+  ${REPO} init ${REPO_URL} -u ${PWD}/.repo/repo -b default -m ${MANIFEST} -g external --mirror
   ${REPO} sync
 }