Easier up debugging of `build-kernel` command

[PLukas2018]

Sometimes there are problems when building kernels to snapshots:

• hanging and waiting for input (as described in #313)
• source not found error message caused by an inability to compile a source file to LLVM IR because of kernel config enabled by default (encountered in a recently done experiment),
• ...

It would be handy if the build-kernel command, when run with the -d option provided more information about its current actions. This should include:

• detailed status updates on the build process,
• outputs of run subcommand.

This should make it easier to identify where problems occur and allow us to fix them faster.