selftests/bpf: Sort configuration

This change makes sure to sort the existing minimal kernel configuration
containing options required for running BPF selftests alphabetically.
Doing so will make it easier to diff it against other configurations,
which in turn helps with maintaining disjunct config files that build on
top of each other. It also helped identify the CONFIG_IPV6_GRE being set
twice and removes one of the occurrences.

Lastly, we change NET_CLS_BPF from 'm' to 'y'. Having this option as 'm'
will cause failures of the btf_skc_cls_ingress selftest.

Signed-off-by: Daniel Müller <deso@posteo.net>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Mykola Lysenko <mykolal@fb.com>
Link: https://lore.kernel.org/bpf/20220727001156.3553701-2-deso@posteo.net