"coverage" build uses different BUILD directory comparing to the normal
build. Previously, any build picked up libaxtls.a from normal build's
directory, but that was fixed recently. So, for each build, we must
build axtls explicitly.
This fixes Travis build in particular.
Unix naming is historical, before current conventions were established.
All other ports however have it as "modusocket.c", so rename for
consistency and to avoid confusion.
This is to keep the top-level directory clean, to make it clear what is
core and what is a port, and to allow the repository to grow with new ports
in a sustainable way.
