eudev: add enable/disable hwdb option

This option enables/disables installation of the eudev hardware database,
which provides various extra properties for known devices.
Enabling this option adds ~5MB to the rootfs size.

The option is enabled by default in order to preserve old behavior.

[Peter: extend commit message]
Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>