udev: convert to virtual package

This patch converts udev to a virtual package. For the moment, there is only
one provider for the udev features: eudev.

Packages meant to provide udev-like features must select the symbol
BR2_PACKAGE_HAS_UDEV.

Packages depending on BR2_ROOTFS_DEVICE_CREATION_DYNAMIC_UDEV or
BR2_PACKAGE_UDEV have been converted to use the new symbol.

[Peter: move legacy symbols under 2014.05]
Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>