In the Cygwin environment the native Windows serial COM# ports get mapped
to /dev/ttyS# for POSIX compatibility. While # is one number lower inside
the environment than the COM port number because it's 0 indexed instead
of 1.
I added the necessary handling to all the dependent parts I found which
allows uploading to /dev/ttyS# when the cygwin platform is detected.
Now the usual "make px4fmu-v4 upload" and
"./Tools/upload.sh build/px4fmu-v4_default/px4fmu-v4_default.px4" work.