So now we either set SETTINGS or CONFIGDIR and if the latter is used
SETTINGS variable is populated from the basename of CONFIGDIR to avoid
having to set two overrides at the same time.
Also add a build.conf.local optional build configuration to make
adjustments beyond our own scope feasible.
o Talking to Shawn Webb revealed that build.conf behaviour is
still fragile. This will properly invoke all build steps with
the variables that are allowed to be overridden.
o Adds an external CONFIG that is allowed to set NAME, VERSION,
FLAVOUR.
o Remove persistent environment awareness.
o Document the whole thing.
There's still some things to do in order to do parallel builds,
but at least now the build scripts can't be poisoned by a changing
config file underneath.