Support for polling wait in shared bus systems

[korken89]

While waiting for the flash memory to complete its work, a lot of time can traverse.
In my case I have a radio IC on the same SPI as the flash memory that logging is happening on, and it would be awesome to be able to do the wait checks in a polling manner.
This would allow for the high priority SPI transactions with, in my case, the radio to happen in parallel with waiting for flash operations.

One way to do this is to use type states and have a Busy state for polling.