Configure the flash as read-only at boot

patches/tock/01-persistent-storage.patch

@@ -1,5 +1,5 @@
 diff --git a/chips/nrf52/src/nvmc.rs b/chips/nrf52/src/nvmc.rs
-index adbc2a2b5..7594ec867 100644
+index adbc2a2b5..4092cf346 100644
 --- a/chips/nrf52/src/nvmc.rs
 +++ b/chips/nrf52/src/nvmc.rs
 @@ -3,15 +3,19 @@
@@ -68,7 +68,7 @@ index adbc2a2b5..7594ec867 100644
              let word: u32 = (data[i + 0] as u32) << 0
                  | (data[i + 1] as u32) << 8
                  | (data[i + 2] as u32) << 16
-@@ -387,3 +399,236 @@ impl hil::flash::Flash for Nvmc {
+@@ -387,3 +399,237 @@ impl hil::flash::Flash for Nvmc {
          self.erase_page(page_number)
      }
  }
@@ -139,6 +139,7 @@ index adbc2a2b5..7594ec867 100644
 +        apps: Grant<App>,
 +        deferred_caller: &'static DynamicDeferredCall,
 +    ) -> SyscallDriver {
++        nvmc.configure_readonly();
 +        SyscallDriver {
 +            nvmc,
 +            apps,