HOB:Proper handle of SIGINT

Modal dialogs doesn't run on the main loop so they cannot
catch any signal from the terminal. This patch makes sure
the dialogs are destroyed when a SIGINT is sent to HOB.

[YOCTO #3329]

Signed-off-by: Valentin Popa <valentin.popa@intel.com>
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>