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.

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
systemd: bump to v207.

This patch bumps systemd to v207 but also declares it as a provider for the
udev virtual package.

Starting with systemd 183, udev has been merged into
systemd. The udev daemon is now installed as /lib/systemd/systemd-udevd.
This means that /dev management using udev is only available if systemd
is chosen as init system.

When configuring systemd, the following options are available:

- activation of systemd-journal-gatewayd, to access the journal via
  HTTP.
- activation of extra features like journal compression and sealing.

Support for uClibc has also been removed because:

- upstream has no interest in supporting uClibc.
- using a shrinked libc brings no advantage, given the size of all the
  programs included in Systemd. So using glibc does not matter.

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
manual: update /dev management section.

This patch updates the /dev management section in the manual with information
about eudev, which replaces udev.
board: add elebihan demo-systemd.

New config files are available to build a kernel and file system to showcase
the use of systemd, with QEMU: one for x86 and the other for ARM.

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
board: add elebihan demo-eudev.

New config files are available to build a kernel and file system to showcase
the use of eudev, with QEMU: one for x86 and the other for ARM.

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>