SD card support

This adds SD card support to:

1. st3m, providing st3m_fs_sd_* functions and automatically attempting to probe the SD card on st3m_fs_init().
2. Recovery mode, providing SD card Disk Mode
3. The main firmware, attempting to mount the card to /sd on startup.

main/fs.c

@@ -2,6 +2,7 @@
 
 #include "st3m_fs.h"
 #include "st3m_fs_flash.h"
+#include "st3m_fs_sd.h"
 #include "st3m_mode.h"
 #include "st3m_sys_data.h"
 #include "st3m_tar.h"
@@ -60,4 +61,9 @@ void flow3r_fs_init(void) {
         ESP_LOGI(TAG, "No %s on flash, preparing sys directory...", sysflag);
         _extract_sys_data();
     }
+
+    esp_err_t ret = st3m_fs_sd_mount();
+    if (ret != ESP_OK) {
+        ESP_LOGW(TAG, "Failed to mount SD card: %s", esp_err_to_name(ret));
+    }
 }