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.

components/st3m/CMakeLists.txt

@@ -19,6 +19,7 @@ idf_component_register(
         st3m_tar.c
         st3m_fs.c
         st3m_fs_flash.c
+        st3m_fs_sd.c
     INCLUDE_DIRS
         .
     REQUIRES