nova: Remove Ec::cont to make thread switching easier to understand

In the full NOVA hypervisor, Ec::cont was used to implement
continuation-style switching of execution contexts. For our simplified
version, it is not needed, but it must be understood to correctly use
Ec::make_current(), which many students consider confusing. Therefore,
we remove Ec::cont and make Ec::make_current() to always jump to
Ec::ret_user_sysexit().

As a consequence of Ec::cont removal, this commit also reworks root
task initialization and invocation. This also makes the program flow
during boot easier to understand.

[Code and commit message modified by M. Sojka]