Conflicting dependencies yields inconsistent dependency graph

[ccollins476ad]

syscfg.vals:
    FOO: 1

syscfg.vals.BAR:
    FOO: 1

These two syscfg.vals mappings are in conflict. The first enables FOO unconditionally; the second only enables FOO if bar is set. When newt encounters a conflict like this, it accepts the first override it sees and discards the others. Since these overrides are not processed in a consistent order, the result varies from build to build.

Newt should not allow such conflicting overrides. Initially, we can make newt emit a warning when it encounters this. Later, we can change it to an error.