Expose freebytes too

lib/hal/HalStorage.cpp

@@ -81,6 +81,8 @@ uint64_t HalStorage::sdUsedBytes() const {
   return (clusterCount - cappedFreeClusters) * bytesPerCluster;
 }
 
+uint64_t HalStorage::sdFreeBytes() const { return sdTotalBytes() - sdUsedBytes(); }
+
 class HalFile::Impl {
  public:
   Impl(FsFile&& fsFile) : file(std::move(fsFile)) {}

lib/hal/HalStorage.h

@@ -47,6 +47,7 @@ class HalStorage {
 
   uint64_t sdTotalBytes() const;
   uint64_t sdUsedBytes() const;
+  uint64_t sdFreeBytes() const;
 
   static HalStorage& getInstance() { return instance; }