triggerhappy: new package

[Thomas:
  - add BR2_USE_MMU dependency, since fork() is used
  - rename do_start() and do_stop() to just start() and stop(), as we
    do in most init scripts in Buildroot.]

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>