makedevs: add capability support

makedevs: add capability support

Add the support of capability to makedevs as extended attribute.
Now, it's possible to add a  line "|xattr <capability>" after a
file description to also add a capability to this file. It's
possible to add severals capabilities with severals lines.

[Peter: extend doc, reword Config.in, extend error message,
	use HOST_MAKEDEVS_CFLAGS/LDFLAGS for all flags]
Signed-off-by: Philippe Reynes <philippe.reynes@sagemcom.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>