Add an early boot service that exports the microSD block device over
USB mass storage before the filesystem and normal flight application
start. The service retains exclusive ownership until a power cycle and
continues servicing the watchdog.
Enable the service on supported boards, provide explicit build control,
increase the MSD worker stacks for the SD wait path, scope the ChibiOS
fixed-width serial warning suppression to the MSD object, and reject
explicit enable requests on unsupported boards.
