Not sure why they got removed in the first place in the merge.
I also don't know how they are used and what they are for, comments
welcome.

There is no such thing as set_param_no_autosave on QURT, therefore just
save it anyway. On the Snapdragon the overhead should not be a problem.

Otherwise SITL doesn't work anymore, at least on Linux

This mainly fixes the NuttX build.

Don't try to open the mavlink_fd on QURT because the px4_ioctl leads to
timeouts of the attitude_estimator_q loop.

mainapp was not restoring the original terminal setttings on exit

Signed-off-by: Mark Charlebois <charlebm@gmail.com>

Like this the timeouts are going through instead of hanging everything.

It was wrong to subtract seconds and nanoseconds from each other. This
will wrap, therefore use ts_to_abstime and do the subtraction
afterwards.

The timeout was triggered using absolute time instead of a delay in
usec. This lead to the system hanging. With the fix it continues after
the timeout, however, the rates still don't seem right.