Add environment.d hook to setup environment.

Before each phase dib-run-parts will pick these up to setup the
environment for the commands it runs.

Change-Id: I56697d7bc370918d4063eee67a1c35d4988a5359

README.md

@@ -169,6 +169,9 @@ part of the process you need to customise:
 * first-boot.d: Runs inside the image before rc.local. Scripts from here are
   good for doing per-instance configuration based on cloud metadata.
 
+* environment.d: Bash script snippets that are sourced before running scripts
+  in each phase. Use this to set an environment variable for other hooks.
+
 * element-deps : A plain text, newline separated list of elements which will
   be added to the list of elements built into the image at image creation time.
 

elements/dib-run-parts/root.d/dib-run-parts

@@ -49,6 +49,12 @@ targets=$(find $target_dir -type f -executable -printf '%f\n' | grep -E "$allowe
 
 PROFILE_DIR=$(mktemp -d /tmp/profiledir.XXXXXX)
 
+if [ -d /tmp/in_target.d/environment.d ] ; then
+    for env_file in /tmp/in_target.d/environment.d/* ; do
+        source $env_file
+    done
+fi
+
 for target in $targets ; do
     output "Running $target_dir/$target"
     target_tag=${target//\//_}