dev/uart: Use a linker set to find uart classes

[andrew]

Summary

When the uart is configured via the environment we need to find the
uart class with a specified name. Currently to do this with an
incomplete list of uarts. As we may not have included all uarts in the
kernel each class is defined as weak.

Switch to a linker set so the list is always up to date based on what
is included in the kernel, and the class can be static.

Sponsored by: Arm Ltd

[imp]

I've stumbled over this in the past when I've not had the time to fix. Thanks!

[bz]

I've head a slightly more elaborate version of this from @jhb since 2019.
I'l add @jhb to Cc:
Also I'll try to upload the diff as an attachment so you can have a look.

side effect: this removes weak symbols showing up in backtraces of panic if going through 0.{F74534184}