Add extensive error checking that will stop the build on first failure.

This revealed a couple of stampinf invocations on inf files that no
longer exist, so also remove those stampinf invocations.

Do not do git clone and checkout if xc-vusb.git already exists.
This makes incremental builds much faster. If we do use git, use the
version passed in via a gitbin argument rather than just picking up
whatever version happens to be on the path. Ideally we'd use the
logic in xc-windows.git/winbuild-all.ps1 to do the checkout, once
someone has time to reorganise the build tree.

Add a comment that refers people to
https://github.com/OpenXT/xc-windows/issues/3 which explains why we
still have to do clean builds of the DDK stuff even though I'm aiming
for fast incremental builds. The real fix is to run something like
an rsync to a copy of the source code, and build that instead, to keep
source and each type of binary file separate.