Fixes for VS2015/WDK10 build

Fixes for VS2015/WDK10 build

The package build was not working correctly and caused the overall build
to fail.
At least part of the reason for this is that Microsoft, in their infinite
wisdom, have removed the DIFx redist from WDK10. This patch makes use of
a new environment variable 'DPINST_REDIST' to find the copy of dpinst.exe
to package such that this can be pointed at an older WDK or alternative
location where dpinst.exe can be found.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>