virtual flight mode switch & 6 flight modes

some flight controls like e.g. APM/Arducopter support a single channel
that can have 6 different states used to switch flight modes.
this is what virtual flight mode switch does.
using the channel layout field within the model profile we can now
define one channel with the letter M. (M...Mode Switch).
When doing so, in addition to the already configured flight mode switch,
also the bi-state switch SW1 will be evaluated and combined with the
flight mode switch. resulting from that 3 additional flight modes can be
defined and moreover, the M-channel will then have one of the values
from the cfg_VFMSteps list of values according to the flight mode.
a typical channel layout for APM would then look like: "AETRMP2-"
serving AIL, ELE, TRH, RUD, VIRTUAL MODE SWITCH (SW1/SW3), POT1, SW2,
while keeping the last channel free to assign something individual like
another switch for example.
if there is no M-channel in the channel layout, only 3 flight modes will
be selectable, meaning only the first 3 DR/EXPO values will be
evaluated.
NOTE: change in RC library Channel.cpp was reverted again, since we
solved that in the t5x sketch directly...