flash_simulator: add ability to use memory region

Add the ability for the flash simulator to store its contents in a
memory region.

This allows filesystems on the flash simulator to survive a reboot.
And allows subsystems (e.g. coredump) to store their info on ram while
using the (existing) flash partition backend.

Add a example (for nucleo_f411re) that shows how to configure the flash
simulator for hardware (cfg discussion #54166).

Signed-off-by: Laczen JMS <laczenjms@gmail.com>