Add support for SpiNorFlash and FS (#30)

The external SPI flash is implemented as a 4MB on the local filesystem.
This allows the FS (littleFS) and settings to work properly.

Remove the simulated `FS.h` and `FS.cpp`, because we can now use
the files from InfiniTime directly as the heavy lifting is done in the simulated
`SpiNorFlash.h` and cpp files.

`SpiNorFlash.h` provides read and write functions with `uint8_t` buffer, but
`fs::fstream` expects `char` buffer. Use `reinterpret_cast` and check if by
any chance the `char` type on a platform is implemented with more
than one byte. Then the `reinterpret_cast<char *>(buffer)` would change the
meaning of the `size` parameter, which could lead to garbage data.

Co-authored-by: Reinhold Gschweicher <pyro4hell@gmail.com>

sim/drivers/SpiNorFlash.h

@@ -1,13 +1,15 @@
 #pragma once
 #include <cstddef>
 #include <cstdint>
+#include <fstream>
 
 namespace Pinetime {
   namespace Drivers {
     class Spi;
     class SpiNorFlash {
     public:
-      explicit SpiNorFlash(Spi& spi);
+      explicit SpiNorFlash(const std::string& memoryFilePath);
+      ~SpiNorFlash();
       SpiNorFlash(const SpiNorFlash&) = delete;
       SpiNorFlash& operator=(const SpiNorFlash&) = delete;
       SpiNorFlash(SpiNorFlash&&) = delete;
@@ -53,8 +55,12 @@ namespace Pinetime {
       };
       static constexpr uint16_t pageSize = 256;
 
-      Spi& spi;
+      static constexpr size_t memorySize {0x400000};
+      const std::string& memoryFilePath;
+
+
       Identification device_id;
+      std::fstream memoryFile;
     };
   }
 }