hdt: Adding postexec= option

hdt: Adding postexec= option

When HDT is exiting, you might need executing something else.
This could be used in the following scenario :

You start HDT, do an automatic command like 'dump; exit', but then after
you might need to launch something else from syslinux.

The postexec option will allow you to define what label you'd love
running one HDT got terminated.

Syntaxt is like the following:

postexec='menu_label_to_run_once_hdt_got_exited'

Note the quotes (') after the equal sign (=)

This could looks like :
APPEND auto='dump; exit' postexec='memtest'