bsd.confs.mk: Support CONFGROUPS.yes

Align CONFGROUPS with other options like SUBDIRS and FILESGROUPS by
supporting the CONFGROUPS.yes option. This means logic like this:

.if ${MK_FOO} != "no"
CONFGROUPS+= FOO
.endif

can be replaced by:

CONFGROUPS.${MK_FOO}+= FOO

MFC after: 1 week
Reviewed by: imp
Sponsored by: https://www.patreon.com/bsdivy
Differential Revision: https://reviews.freebsd.org/D52923