Add a post-image script mechanism

Just like we have a post-build script mechanism that gets executed
after the build of all packages but before the creation of the
filesystem images, let's introduce a post-image script mechanism, that
gets executed once all filesystem images have been generated.

This can for example be used to call a tool building a firmware image
from different images generated by Buildroot, or automatically extract
the tarball root filesystem image into some location exported by NFS,
or any other custom action.

[Peter: fix image script check]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: Luca Ceresoli <luca@lucaceresoli.net>
Acked-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>