Make log level configurable from config

[ALSchwalm]

Currently we set the log level in kmain: https://github.com/mythril-hypervisor/mythril/blob/master/mythril/src/kmain.rs#L189

We should allow this to be configured at run time. I'm open to suggestions about how to deal with the early messages (prior to parsing the cfg).

[smaydew]

I'd like to give this one a shot.

[ALSchwalm]

Sounds good! I've assigned you