driver::storage: Fix isSliceUninitialized() for AVREEPROM

`kaleidoscope::driver::storage::AVREEPROM` wasn't implementing its own
`isSliceUninitialized()` method, and relied on the Base class to do so. However,
since we're not using `virtual` methods, the base class was using
`Base::read()` (which always returns 0) rather than `AVREEPROM::read()`, so
`isSliceUninitialized()` always returned false.

To fix this, we implement the function in `AVREEPROM`, and let the default
implementation in Base always return false.

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

src/kaleidoscope/driver/storage/AVREEPROM.h

@@ -56,6 +56,14 @@ class AVREEPROM : public kaleidoscope::driver::storage::Base<_StorageProps> {
   void update(int idx, uint8_t val) {
     EEPROM.update(idx, val);
   }
+
+  bool isSliceUninitialized(uint16_t offset, uint16_t size) {
+    for (uint16_t o = offset; o < offset + size; o++) {
+      if (this->read(o) != _StorageProps::uninitialized_byte)
+        return false;
+    }
+    return true;
+  }
 };
 
 }  // namespace storage

src/kaleidoscope/driver/storage/Base.h

@@ -50,11 +50,7 @@ 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;
+    return false;
   }
 
   const uint16_t length() {