pmb: config: Require CONFIG_CGROUPS=y kconfig option

elogind requires cgroup support in the kernel. It does not start without it, resulting into errors like:

elogind-daemon[1654]: Failed to mount tmpfs at /sys/fs/cgroup: No such file or directory
elogind-daemon[1654]: Failed to mount cgroup at /sys/fs/cgroup/elogind: No such file or directory
elogind-daemon[1654]: Failed to allocate manager object: No such file or directory

dbus-daemon[1437]: [system] Activating service name='org.freedesktop.login1' requested by ':1.1' (uid=0 pid=1642 comm="/usr/bin/lightdm ") (using servicehelper)
dbus-daemon[1437]: [system] Failed to activate service 'org.freedesktop.login1': timed out (service_start_timeout=25000ms)

Most of the supported desktop environments (e.g. Weston) do not start without elogind, so we should require CONFIG_CGROUPS=y in all kernels to avoid this problem.

New section in wiki: https://wiki.postmarketos.org/wiki/Kernel_configuration#CONFIG_CGROUPS

pmaports MR: https://gitlab.com/postmarketOS/pmaports/merge_requests/724