Change the documentation to match current information.

Changed udev references to point to commits in systemd git tree.

Signed-off-by: Marko Saukko <marko.saukko@jolla.com>
Added more configs required by systemd.

[config] CONFIG_FW_LOADER_USER_HELPER, CONFIG_TIMERFD, CONFIG_EPOLL systemd requirements.

Signed-off-by: Marko Saukko <marko.saukko@jolla.com>
systemd-bootchart requirements as optional

[config] CONFIG_SCHEDSTATS and CONFIG_SCHED_DEBUG required by systemd-bootchart.

Signed-off-by: Marko Saukko <marko.saukko@jolla.com>