Move choice between prod and test storage to embedded_flash module

This way all users of storage can share the logic to choose between flash or RAM
storage depending on the "std" feature. This is needed because the store_latency
example assumes flash storage but is built when running `cargo test
--features=std`.

src/embedded_flash/mod.rs

@@ -17,3 +17,36 @@ mod syscall;
 
 #[cfg(not(feature = "std"))]
 pub use self::syscall::SyscallStorage;
+
+/// Storage definition for production.
+#[cfg(not(feature = "std"))]
+mod prod {
+    pub type Storage = super::SyscallStorage;
+
+    pub fn new_storage(num_pages: usize) -> Storage {
+        Storage::new(num_pages).unwrap()
+    }
+}
+#[cfg(not(feature = "std"))]
+pub use self::prod::{new_storage, Storage};
+
+/// Storage definition for testing.
+#[cfg(feature = "std")]
+mod test {
+    pub type Storage = persistent_store::BufferStorage;
+
+    pub fn new_storage(num_pages: usize) -> Storage {
+        const PAGE_SIZE: usize = 0x1000;
+        let store = vec![0xff; num_pages * PAGE_SIZE].into_boxed_slice();
+        let options = persistent_store::BufferOptions {
+            word_size: 4,
+            page_size: PAGE_SIZE,
+            max_word_writes: 2,
+            max_page_erases: 10000,
+            strict_mode: true,
+        };
+        Storage::new(store, options)
+    }
+}
+#[cfg(feature = "std")]
+pub use self::test::{new_storage, Storage};