rt: Allow concurrent Shell:block_on calls

[LucioFranco]

Related to #2720

This makes Shell::block_on go from a &mut self to &self and follows a similar pattern to #2804. Similar to the basic_scheduler this will wrap the driver in a mutex and allow a single thread to park the driver. Once, that thread is done with the parker it will replace it back into the mutex state and it will use Notify to wake up other threads to then attempt to steal the driver.