Prior to this commit, only sector 0 was erase/write protected, which may
not be enough to protect all of mboot (especially if mboot lives at a
higher address than the start of flash).
This commit makes sure all internal flash sectors that mboot lives in are
protected from erasing and writing. The linker script must define
_mboot_writable_flash_start for this to work.
Signed-off-by: Damien George <damien@micropython.org>
771c16f3d9