More systemd config options.

[config] CONFIG_TMPFS_XATTR and CONFIG_SECCOMP are optional for systemd.

Did also modify some notes so that they point to the real information.

Signed-off-by: Marko Saukko <marko.saukko@jolla.com>
Update connman kernel config requirements.

[config] Update conman kernel config requirements.

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