scons: allow use of current builds as default build settings

Currently the --default= option only looks at the predefined
build configs (in m5/build_opts), so you're limited to basing
a new build config off of those (ALPHA_SE, etc.).  If you've
already defined a non-standard build config and want to clone
it or tweak it, you have to start from scratch.  This patch
causes --default= to look first among the existing builds
(in build/variables) before looking in build_opts so you
can specify an existing non-standard build config as a
starting point for a new config.