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>

Variables.mk

@@ -97,3 +97,6 @@ export GOPROXY
 
 # built tags needed for wwbuild binary
 WW_GO_BUILD_TAGS := containers_image_openpgp containers_image_ostree
+
+Defaults.mk:
+	printf " $(foreach V,$(VARLIST),$V := $(strip $($V))\n)" >Defaults.mk