The SLPI drivers dsp_hitl and mavlink_rc_in hardcoded an include path
into the voxl2-default build output for MAVLink headers. This created
an undeclared cross-target dependency that required voxl2-default to be
built first.
Generate the MAVLink common dialect headers directly during the SLPI
build using mavgen.py, outputting to the SLPI build's own directory.
An INTERFACE library (mavlink_common_headers) propagates the include
paths and warning suppression flags to both drivers.
Signed-off-by: Ramon Roche <mrpollo@gmail.com>