Add support for custom post-build script

The config help text says it all:

  Specify a script to be run after the build has finished and before
  the BR2 starts packing the files into selected packages.

  This gives users the oportunity to do board-specific cleanups,
  add-ons and the like, so the generated files can be used directly
  without further processing.

  The script is called with the target directory name as first and
  only argument. Make sure the exit code of that script is 0,
  otherwise make will stop after calling it.

Signed-off-by: Daniel Mack <daniel@caiaq.de>
Cc: Sven Neumann <s.neumann@raumfeld.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>