nvme: add optional udev support

nvme: add optional udev support

Commands like 'nvme list' require libudev but support for libudev is
decided at compile time, and in buildroot this is hard disabled.
Add a check for udev and if it is available build nvme with udev
support.

Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>
[Thomas:
 - add comment to explain the LIBUDEV variable value
 - use += instead of = to assign <pkg>_MAKE_OPTS.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>