build.sh: user friendlier default behavior

build.sh has become the central construction tool for the Patmos toolchain and hardware. @rbscloud remarked that building bench has become quite the burden for a new user, who wants to try Patmos.

This change affects the default build targets and how they are overridden in build.cfg. It also makes -a an option to build *all the targets*.

Note: It is currently assumed that everyone creates a build.cfg from the template.