Refactor Makefile config / Defaults.mk

Makefile has included a Defaults.mk file to store variable configuration
from one run for implicit inclusion in future runs, but the creation of
this file has been automatic and opaque, and the state of it managed by
a series of PHONY targets (e.g., config, genconfig, rm_config). This
commit refactors config to name its files explicitly (so they can be
more properly monitored by make) and makes the generation of Defaults.mk
explicit so that its behavior is more obvious.

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

userdocs/quickstart/el8.rst

@@ -13,7 +13,7 @@ Install Warewulf and dependencies
 
    git clone https://github.com/hpcng/warewulf.git
    cd warewulf
-   make genconfig \
+   make clean Defaults.mk \
     PREFIX=/usr \
     BINDIR=/usr/bin \
     SYSCONFDIR=/etc \

userdocs/quickstart/el9.rst

@@ -14,7 +14,7 @@ Install Warewulf and dependencies
 
    git clone https://github.com/hpcng/warewulf.git
    cd warewulf
-   make genconfig \
+   make clean Defaults.mk \
     PREFIX=/usr \
     BINDIR=/usr/bin \
     SYSCONFDIR=/etc \

userdocs/quickstart/suse15.rst

@@ -16,7 +16,7 @@ Install Warewulf and dependencies
 
    git clone https://github.com/hpcng/warewulf.git
    cd warewulf
-   PREFIX=/usr SYSCONFDIR=/etc TFTPDIR=/srv/tftproot LOCALSTATEDIR=/var/lib make genconfig
+   PREFIX=/usr SYSCONFDIR=/etc TFTPDIR=/srv/tftproot LOCALSTATEDIR=/var/lib make clean Defaults.mk
    make all
    sudo make install