Remove the explicit build and build-indep targets

Mixing these explicit targets with overrides was problematic,
e.g. there was no guarantee that the build-source-stamp target would
be processed after the quilt and autoreconf dh sequences, that worked
only by chance.

To avoid doing the main and udeb builds twice in a full (binary-arch +
binary-indep) build, I have decided to do what is different for them
in dh_autoreconf overrides.  Since dh_autoreconf does only run once,
this avoids extra work.