[Buildroot] [PATCH v2] Post-image script feature and other related fixes

[Thomas Petazzoni]

Hello,

As discussed during the Buildroot meeting, here is a new version of
the post-image thing and the removal of the customize package.

The number of changes is fairly light:
 * Fixed a few typos in the documentation
 * Added BR2_PACKAGE_CUSTOMIZE in Config.in.legacy
 * Removed the special handling of the customize package from
   pkg-stats.

Best regards,

Thomas