pc_serial: trace when ignoring the kernel debug port.

* Since it is currently ignored even when serial debug is disabled in
the settings file, this was a bit confusing.
Storage Kit tests: apply relevant parts of #7025

Patch by Karvjorm. Picked only the code changes as it seems the Jamfile
changes were not needed or already done.
* Use BObjectList instead of BList
* Add some more directory constants to FindDirectory tests.
BootManager: different error when no MBR is found

We were using the same "no space available" message used when the first
partition starts too early. Be more specific here to make it clearer
what the problem is.

Fixes #7087.