udevadm: support updating hwdb from an offset root

Introduce `--root` option to make users run udevadm on rootfs not
mounted as /

Ease the life of distribution packagers.
Merge pull request #44 from lu-zero/master

add --root to support generating hwdb in offsetted rootfs

People using DESTDIR to prepare packages had issues with the hardware database. We add an autotools option to handle that.

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
Signed-off-by: Richard Yao <ryao@gentoo.org>