Add a new target to touch the ${.OBJDIR}/release file, which
indicates the 'release' target has run (in order to prevent
subsequent invocations that may clobber original build output).

As is, the 'release' target is a dummy target that does nothing
more than depend on subsequent targets. Unless 'make obj' is
invoked prior to 'make release', .OBJDIR and .CURDIR will always
be '/usr/src/release' (or wherever /usr/src is located).

When 'make release' invokes 'make real-release' (and subsequent
targets), .OBJDIR is not updated, which still leads to src/ tree
pollution.

While arguably a hack, 'make release' will invoke the original
dummy targets as originally intended, but instead of touching an
empty file (or returing @true), will call a 'release-done' target
that will trigger the behavior that was intended to prevent
a subsequent invocation.

Discussed with: hrs
MFC after: 3 days
X-MFC-With: r293173
Sponsored by: The FreeBSD Foundation