setup the MODESWITCH channel as our OVERRIDE_CHAN, allowing for
instant manual passthru in px4io when OVERRIDE_CHAN goes above 1750
This makes for faster switching, and avoids bugs where a still enabled
FMU channel disables override
First bug is going via microSD to poulate mixer. We can't rely on the
microSD being writeable or functioning properly. Instead create the
buffer in memory and only write a copy to the filesystem.
Second bug is related to extreme trim values on channels. If trim
values are well out of range then the mixer fails and override fails.
Andrew Tridgell