build: Add trivial out-of-tree build support

Usage:

make O=path/to/output

The default O= is ./bin

Signed-off-by: Jason S. McMullan <jason.mcmullan@gmail.com>
xvcd: Allow specification of network port, USB Vendor and USB Product

Signed-off-by: Jason S. McMullan <jason.mcmullan@gmail.com>
Merge pull request #1 from ezrec/master

xvcd (minor improvements)