manual: add information about INSTALL_IMAGES_CMDS

The Buildroot manual currently does not contain any mention of the
<pkg>_INSTALL_IMAGES_CMDS which could potentially be of interest when
developing a new 'generic-package'.

Signed-off-by: Ryan Barnett <rjbarnet@rockwellcollins.com>
CC: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>