package/systemd: remove BR2_PACKAGE_SYSTEMD_ALL_EXTRAS

The option was a bit misleading since it only selected additional
libraries without --enable/disabling any features. Including all of them
here would add a lot of bloat so instead, it is replaced with notes
detailing which features each library enables.

Signed-off-by: Gabe Evans <gabe@hashrabbit.co>
[Thomas:
 - slightly tweak Config.in help text, as suggested by Arnout.
 - add Config.in.legacy handling for BR2_PACKAGE_SYSTEMD_ALL_EXTRAS,
   as suggested by Arnout.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>