fixes for compiling with gcc 10

gcc 10 no longer lets you get away with not externing global variables in header files. This adds the necessary extern and adds defines in appropriate c files

bootloader/storage/nx_sd.c

@@ -25,6 +25,11 @@
 static bool sd_mounted = false;
 static u32  sd_mode = SD_UHS_SDR82;
 
+
+sdmmc_t sd_sdmmc;
+sdmmc_storage_t sd_storage;
+FATFS sd_fs;
+
 u32 sd_get_mode()
 {
 	return sd_mode;