fix(sdcard): use init marker in initialised BSS segment (#2517)

`g_FATFS_Obj` is located in `__DMA` section, which is not initialised, so that testing `sdMounted()` at boot relies on the fact that the bootloader linker script declares BSS in internal SRAM and not in CCM, as the firmware linker script does.

radio/src/targets/horus/board.cpp

@@ -182,10 +182,6 @@ void boardInit()
 
   audioInit();
 
-  // we need to initialize g_FATFS_Obj here, because it is in .ram section (because of DMA access)
-  // and this section is un-initialized
-  memset(&g_FATFS_Obj, 0, sizeof(g_FATFS_Obj));
-
   keysInit();
   rotaryEncoderInit();