Cygwin: repair NuttX build (#8737)

* Cygwin: repaired NuttX build after 1f63d858
all the NuttX specific WINTOOL define handling was skiped in cmake Make.defs generation
The Nuttx build is makfile based and needs its cygwin treatment

Additionally the ${PX4_SOURCE_DIR}/src/include which was added through cmake needs path conversion

The two root causes for the special handling are:
- ARM GCC for Windows doesn't support cygwin paths passed as an argument
  so they need to be either relative or converted via cypath tool
- Symbolic links are totally different under Windows and because NuttX uses them extensively
  it has special handling scripts that need to be fed with the correct defines

* Cygwin: NuttX include paths all converted in Make.defs.in

differentiate between WINTOOL define (MSYS & cygwin) and cygwin using uname command:
- MSYS needs symbolic link handling for ARM GCC but no path conversion
- Cygwin needs both