Use the new memory mutator API to simplify the flash remap code;
this allows us to drop the flash_mapped flag.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Andrzej Zaborowski <andrew.zaborowski@intel.com>