px4io: enable signals

signals will be used to wakeup the mixer on a new set of pwm values
sig_action: fixed uninitialised value
px4io: use SIGUSR1 to wake up mixer on new input

this will reduce the average delay for updating ESCs on multicopters
nsh: added a run_init_scripts flag to nsh_consolemain()

this makes it possible to use nsh_consolemain() to create a new nsh
instance inside another process. Without this an init script failure
(eg. trying to mount a microsd a 2nd time) will cause the new instance
to exit