Adds support for post-install scripts.

Post install scripts are useful because they can perform tasks you want
to handle after the OS/application install but before the first boot
of the image. e.g: Clean the cache left by the package manager reducing
the size of the image.

Change-Id: I03e77f602192bbdce29c02999d1b57fac8051ddc
Fixes: Bug #1145786

README.md

@@ -156,6 +156,12 @@ part of the process you need to customise:
   install packages, chain into configuration management tools or do other
   image specific operations.
 
+* post-install.d: Run code in the chroot. This is a good place to perform
+  tasks you want to handle after the OS/application install but before the
+  first boot of the image. Some examples of use would be: Run chkconfig
+  to disable unneeded services and clean the cache left by the package
+  manager to reduce the size of the image.
+
 * first-boot.d: Runs inside the image before rc.local. Scripts from here are
   good for doing per-instance configuration based on cloud metadata.
 

bin/disk-image-create

@@ -117,6 +117,7 @@ run_d_in_target pre-install
 do_extra_package_install
 # Call install scripts to pull in the software users want.
 run_d_in_target install
+run_d_in_target post-install
 prepare_first_boot
 finalise_base
 unmount_image

elements/dpkg/post-install.d/99-clean-up-cache

@@ -0,0 +1,6 @@
+#!/bin/bash
+# Do an apt-get clean. This will free some space.
+
+set -e
+
+apt-get clean