Makefile.inc1: Let the buildetc target run outside of etc

Commit 49bc071f4088 ("nsswitch.conf: Avoid modification after
installation") changes handling of nsswitch.conf such that we make a
copy in the objdir during a build. Historically, the in-tree
nsswitch.conf lived under etc.

The buildetc target and its copy set SUBDIR_OVERRIDE=etc when building
the object tree, but I think this isn't right when conf files are
scattered around the src tree. If any of them require non-trivial
processing, they'll get skipped during buildetc, and then some
build-time commands may run during installetc. In the linked PR, this
fails because the src tree is mounted read-only and no objdir was
created during buildetc.

Remove the SUBDIR_OVERRIDE for the _obj target, and build the
buildconfig target across the tree.

PR: 286072
Fixes: 49bc071f4088 ("nsswitch.conf: Avoid modification after installation")
Reviewed by: brooks, dim
Tested by: dim, Alastair Hogge <agh@riseup.net>
Differential Revision: https://reviews.freebsd.org/D49960