ARM: omap2plus_defconfig: Fix bloat caused by having ipv6 built-in

Commit  673ce00c5d6c (ARM: omap2plus_defconfig: Add support for
distros with systemd) caused considerable bloat as noted by
Paul Walmsley <paul@pwsan.com>.

Let's fix this issue by making what we can into loadable modules
for the systemd options. That's only IPV6 and AUTOFS4_FS it
seems, and IPv6 defaults to a loadable module.

Reported-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>