Eliminate duplicate afterinstallconfigs target

Define separate dependent targets which afterinstallconfigs relies on, in
order to modify ${DESTDIR}/etc/master.passwd and
${DESTDIR}/etc/nsswitch.conf.

Mark these targets .PHONY, since they manipulate configurations on the fly and
the generation logic isn't 100% defined in terms of the source files/logic,
and is variable, based on MK_foo flags.

MFC after: 2 weeks
Reviewed by: bapt, brd
Differential Revision: https://reviews.freebsd.org/D20330