configs: Add CGROUP_BPF and BPF_SYSCALL

Enabling CONFIG_CGROUP_BPF allows BPF filters to access cgroup-local
storage, which is useful for control network interactions at the
cgroup level. CONFIG_BPF_SYSCALL is required to make use of it.

See: https://github.com/raspberrypi/linux/issues/3294



Signed-off-by: Phil Elwell <phil@raspberrypi.org>