fatfs: Add simple GPT support

This allows for a simple GPT parsing and checking first partition to see if it's FAT based.

This allows hekate booting GPT with tiny size cost.

bdk/storage/nx_sd.h

@@ -51,6 +51,7 @@ bool sd_initialize(bool power_cycle);
 bool sd_mount();
 void sd_unmount();
 void sd_end();
+bool sd_is_gpt();
 void *sd_file_read(const char *path, u32 *fsize);
 int  sd_save_to_file(void *buf, u32 size, const char *filename);