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
Merge "Adds support for post-install scripts."