build: Add support for MSYS2

MSYS2 is a modern fork of Cygwin that is widely supported
and provides frequent releases and good support. It is
also the framework used by the official Windows Git port.
This patch adds MSYS2 support so that builds on Windows using
MSYS2 work properly.

Change-Id: Ia5743a410d1cff983a7aab37f8e3d8228cb8ae8e
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>