bootstrap_sdk: Allow running hooks to modify stage1 repos

Sometimes the modification to stage1 repos is necessary. For example
we will need to update baselayout package in stage1, so we get the
cleaned up handling of PATH and ROOTPATH, which will allow a newer
portage to succeed in calculating dependencies in stage2.

bootstrap_sdk

@@ -158,6 +158,20 @@ stage_repo() {
             && git fetch --all \
             && git checkout "$gitref" )
     fi
+    (
+        set -euo pipefail
+        local repo_var hook name
+
+        # FLAGS_coreos_overlay for gitname coreos-overlay
+        repo_var="FLAGS_${gitname//-/_}"
+        shopt -s nullglob
+        for hook in "${FLAGS_coreos_overlay}/coreos/stage1_hooks/"*"-${gitname}.sh"; do
+            name=${hook##*/}
+            name=${name%"-${gitname}.sh"}
+            info "Invoking stage1 ${gitname} hook ${name} on ${dest}/${repo}"
+            "${hook}" "${dest}/${repo}" "${!repo_var}"
+        done
+    )
 }
 
 build_stage1() {