Updated Makefile to account for vendor/ in dist

The previous PR #890 introduced a `Defaults.mk` target for defining
default variables for future calls to `make`. This functionality was
already present in a different form, but this new "real" target had an
unintended side-effect of causing the target to be called automatically
at reference, even the optional reference `-include Defaults.mk`. This
is undesirable as it causes the generation of defaults even during a
`make clean`.

This commit resolves these issues and further refactors the Makefile,
building on the work in #890, but with additional understanding:

* Convert the `Defaults.mk` target to the "phony" `defaults` target,
  preventing automatic generation
* Refactor and split `clean` targets
* Update `cleanvendor` to reflect `OFFLINE_BUILD`, similar to the
  `vendor` target
* Move "tools" targets into `Tools.mk`
* Add `ARCH` to `Variables.mk` for architecture-specific actions

Signed-off-by: Jonathon Anderson <janderson@ciq.com>

Dockerfile

@@ -7,7 +7,9 @@ RUN zypper  -n install --no-recommends git go1.18 libgpgme-devel &&\
 COPY . /warewulf-src
 
 RUN cd /warewulf-src &&\
-  make contclean Defaults.mk \
+  make clean \
+    OFFLINE_BUILD=1 &&\
+  make defaults \
     PREFIX=/usr \
     BINDIR=/usr/bin \
     SYSCONFDIR=/etc \