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>
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