update-alternatives.bbclass: Disable for mingw32 targets

update-alternatives.bbclass: Disable for mingw32 targets

When building for mingw32 targets (e.g. nativesdk cross compiling for
windows), disable the dependency on update-alternatives as the Windows
platform does not support symlinks or package management.

This avoids the complex (partly non-buildable for mingw32) dependency
chain virtual/update-alternatives -> opkg-utils -> python -> ...

(From OE-Core rev: 0131abe2d94d6836a54bc1616566c3bf3f2d6eb0)

Signed-off-by: Nathan Rossi <nathan@nathanrossi.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>