Renamed ReadOnlyBootPrompt to FirstBootPrompt.

The previous name is no longer applicable, as *BootPrompt is now launched on
the first boot of writable medium (e.g., USB thumb drive and hard drives).
Fixes part of #9399.

Notes:
* BootPrompt.rdef may have lost its history.
* The renaming was done with the following command:
git mv src/apps/readonlybootprompt src/apps/firstbootprompt