MIMIMAL: add uart

While uart could be detected completely through plug and play means, add
it here for two reasons. First, we don't do that from the loader, so
it's not available as a console. Second, even if we did do it from the
loader, there's a limitation in the system today that console drivers
must be compiled into the kernel because the console is selected before
external modules are linked into the kernel. Adding it only increases
the kernel size by ~14k as well.

Sponsored by: Netflix
Idea liked by: des, rpokala, brooks, jhb