Refactor DSi_NAND (#1844)

* Refactor diskio's contents

- Change ff_disk_read_cb/write_cb into a std::function instead of a raw pointer
- Add const specifiers as needed

* Refactor DSi_NAND to manage the file system's mounted lifetime with RAII

* Split NANDMount into NANDMount and NANDImage

- NANDImage is used for information about the NAND that doesn't require decryption or filesystem access
- NANDMount is used to actually access the file system
- Both classes manage their respective resources (the NAND file handle and the NAND's mount) with RAII
- Also split the file loading into another function that I will remove in a later PR

* Make NANDMount immovable

* Remove NAND-loading code that I had sectioned off into a function

- Incomplete copypasta
- I must have gotten distracted

* Tidy up NANDImage's initialization

- Don't unmount the disk image if the constructor fails (that's NANDMount's job now)
- Only assign CurFile if the constructor succeeds

* Add some const-correctness

* Move DSi NAND initialization to the frontend

- The NANDImage is now installed via a unique_ptr in DSi

* Remove Platform::DSi_NANDPath

- Not Config::DSiNANDPath; that can still be configured as usual
- The core no longer needs to care

src/DSi.h

@@ -22,6 +22,11 @@
 #include "NDS.h"
 #include "DSi_SD.h"
 
+namespace DSi_NAND
+{
+    class NANDImage;
+}
+
 namespace DSi
 {
 
@@ -33,9 +38,7 @@ extern u32 SCFG_EXT[2];
 extern u8 ARM9iBIOS[0x10000];
 extern u8 ARM7iBIOS[0x10000];
 
-extern u8 eMMC_CID[16];
-extern u64 ConsoleID;
-
+extern std::unique_ptr<DSi_NAND::NANDImage> NANDImage;
 extern DSi_SDHost* SDMMC;
 extern DSi_SDHost* SDIO;