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/sdmmc_driver.c

@@ -32,6 +32,8 @@
 //#define DPRINTF(...) gfx_printf(__VA_ARGS__)
 #define DPRINTF(...)
 
+u32 sd_power_cycle_time_start;
+
 /*! SCMMC controller base addresses. */
 static const u32 _sdmmc_bases[4] = {
 	0x700B0000,