Log level feature

[asensio-project]

Hello,

This PR adds a log_level configuration in bootloader_api. Now, the developers of the kernel can change which kind of messages are shown in the screen when booting.

Thanks for this project.

(This is the replacement of #292)

Resolves #288

[phil-opp]

Thanks for the PR! Looks good to me overall, but I would prefer to create our own copy of the LevelFilter enum instead of adding a log dependency to the bootloader_api crate (see my comments below).

[asensio-project]

Since d1da62f, this PR doesn't depend in the log crate for the API.

[phil-opp]

Looks good, thanks for the quick update!