Eliminate duplicate `afterinstallconfigs` target

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