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>
Reviewed-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>