driver::storage::Base: Add an isSliceUninitialized() function

First, we add an `uinitialized_byte` prop to `BaseProps`, so implementations
that don't use `0xff` as default can set it to whatever they use. Do keep in
mind that plenty of code still assumes `0xff` is the uninitialized byte, but
this way we have a starting point.

Then, we add an `isSliceUninitialized()` function, which checks whether a given
slice is completely made up from uninitialized bytes or not.

Fixes #1145.

Signed-off-by: Gergely Nagy <algernon@keyboard.io>

src/kaleidoscope/driver/storage/Base.h

@@ -25,6 +25,7 @@ namespace storage {
 
 struct BaseProps {
   static constexpr uint16_t length = 0;
+  static constexpr uint8_t uninitialized_byte = 0xff;
 };
 
 template <typename _StorageProps>
@@ -48,6 +49,14 @@ class Base {
 
   void update(int idx, uint8_t val) {}
 
+  bool isSliceUninitialized(uint16_t offset, uint16_t size) {
+    for (uint16_t o = offset; o < offset + size; o++) {
+      if (read(o) != _StorageProps::uninitialized_byte)
+        return false;
+    }
+    return true;
+  }
+
   const uint16_t length() {
     return _StorageProps::length;
   }