Support binding DSM2 and DSMX satellite receivers

The px4io bind command allows you to put a DSM satellite receiver into
bind mode. Since this feature requires that
the dsm VCC line (red wire) be cut and routed through relay one, it is
not enabled by default in order not to
affect those not using a DSM satellite receiver or wising to use relay
one for other purposes.

NOTE: Binding DSM2 satellites in 11-bit mode is not supported due to
potential bug in some DSM2 receiver streams
when in 11-bit mode. Furthermore the px4io software folds 11 bit data
down to 10 bits so there is no resolution
advantage to to 11-bit mode.

To enable the feature the RC_RL1_DSM_VCC parameter must be set to a non
zero value from the console, or using
QGroundControl:

param set RC_RL1_DSM_VCC 1

From the console you can initiate DSM bind mode with:

uorb start
param set RC_RL1_DSM_VCC 1
px4io start
px4io bind dsm2

For binding a DSMX satellite to a DSMX transmitter you would instead
use:

px4io bind dsmx

Your receiver module should start a rapid flash and you can follow the
normal binding sequence of your
transmitter.

Note: The value of parameter RC_RL1_DSM_VCC defaults to 0, so none of
this will have any effect on an unmodified
DSM receiver connection. For this feature to work, the power wire (red)
must be cut and each side connected to a
terminal on relay1 of the px4io board.

This has been tested using Spektrum as well as Hobby King 'Orange' DSM
satellite receivers.

Both px4fmu and px4io images are updated.
Merge remote-tracking branch 'upstream/master'
Merge remote-tracking branch 'upstream/master'
Prevent RELAY1 control via IOCTL if DSM bind feature is enabled
Minor px4io optimization

Since this module creates the PX4IO object and that the IOCTL function
doesn't use the file descriptor parameter, there is no need to invoke
IOCTL via the filesystem since we can call it directly.
Feature documentation
Fix documentation fonts
Merge remote-tracking branch 'upstream/master'
Merge remote-tracking branch 'upstream/master'
Merge remote-tracking branch 'upstream/master'
General cleanup of /dev/px4io and /dev/px4fmu

- Use distinct common symbols for px4io and px4fmu device files, and use
instead of hardcoded filenames
- Use common symbols defining px4io bits consistently between px4fmu and
px4io builds.