release: Use in-tree etcupdate for build

This is in preparation for non-FreeBSD and -DNO_ROOT builds. On
non-FreeBSD there is no host etcupdate to use, and -DNO_ROOT will
require additional flags that may not be supported by the host's
etcupdate when building on FreeBSD. Moreover, there's no guarantee
anyway that the host's etcupdate is quite right for the current tree;
upgrading from source only requires that the host's is good enough for
-p which just manually copies master.passwd and group, the rest of the
upgrade is done post-installworld. For example, should a new set of
autogenerated files be added that etcupdate is taught about, the host
won't know about them and so the bootstrapped current tree will
incorrectly contain them, leading to spurious diffs on the installed
system.

Reviewed by: gjb, delphij
Obtained from: CheriBSD
Differential Revision: https://reviews.freebsd.org/D35853