drivers/flash/flash_simulator: erase posix flash in initialization

After startup the native_posix flash was filed by 0x00 instead
of 0xff, which is not coherent with erased byte value which
is 0xff for the native_posix target.

Such inconsistent behavior makes for instance running
a storage system designed(or configured) for a flash device
with 0xff as erased byte values problematic.

This patch makes flash erased after new flash device file creation.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>