stmhal: Add retry to SD card init.

This fixed an issue with a certain SD card sometimes not initialising
first time round.  See issue #822 for related, and thanks to
@iabdalkader for the idea.

stmhal/sdcard.c

@@ -105,11 +105,14 @@ bool sdcard_power_on(void) {
     sd_handle.Init.HardwareFlowControl = SDIO_HARDWARE_FLOW_CONTROL_DISABLE;
     sd_handle.Init.ClockDiv            = SDIO_TRANSFER_CLK_DIV;
 
-    // init the SD interface
+    // init the SD interface, with retry if it's not ready yet
     HAL_SD_CardInfoTypedef cardinfo;
-    if (HAL_SD_Init(&sd_handle, &cardinfo) != SD_OK) {
+    for (int retry = 10; HAL_SD_Init(&sd_handle, &cardinfo) != SD_OK; retry--) {
+        if (retry == 0) {
             goto error;
         }
+        HAL_Delay(50);
+    }
 
     // configure the SD bus width for wide operation
     if (HAL_SD_WideBusOperation_Config(&sd_handle, SDIO_BUS_WIDE_4B) != SD_OK) {