stm32/mboot: Add ELEM_TYPE_STATUS element so application can get status.

This new element takes the form: (ELEM_TYPE_STATUS, 4, <address>).  If this
element is present in the mboot command then mboot will store to the given
address the result of the filesystem firmware update process.  The address
can for example be an RTC backup register.
Signed-off-by: Damien George <damien@micropython.org>

ports/stm32/boards/stm32f4xx_hal_conf_base.h

@@ -54,6 +54,7 @@
 #include "stm32f4xx_hal_usart.h"
 #include "stm32f4xx_hal_wwdg.h"
 #include "stm32f4xx_ll_adc.h"
+#include "stm32f4xx_ll_pwr.h"
 #include "stm32f4xx_ll_rtc.h"
 
 // Enable various HAL modules

ports/stm32/boards/stm32f7xx_hal_conf_base.h

@@ -54,6 +54,7 @@
 #include "stm32f7xx_hal_usart.h"
 #include "stm32f7xx_hal_wwdg.h"
 #include "stm32f7xx_ll_adc.h"
+#include "stm32f7xx_ll_pwr.h"
 #include "stm32f7xx_ll_rtc.h"
 
 // Enable various HAL modules

ports/stm32/boards/stm32h7xx_hal_conf_base.h

@@ -54,6 +54,7 @@
 #include "stm32h7xx_hal_usart.h"
 #include "stm32h7xx_hal_wwdg.h"
 #include "stm32h7xx_ll_adc.h"
+#include "stm32h7xx_ll_pwr.h"
 #include "stm32h7xx_ll_rtc.h"
 
 // Enable various HAL modules

ports/stm32/mboot/main.c

@@ -1443,7 +1443,14 @@ enter_bootloader:
         // Application passed through elements, validate then process them
         const uint8_t *elem_end = elem_search(ELEM_DATA_START, ELEM_TYPE_END);
         if (elem_end != NULL && elem_end[-1] == 0) {
-            fsload_process();
+            int ret = fsload_process();
+            // If there is a valid ELEM_TYPE_STATUS element then store the status in the given location.
+            const uint8_t *elem_status = elem_search(ELEM_DATA_START, ELEM_TYPE_STATUS);
+            if (elem_status != NULL && elem_status[-1] == 4) {
+                uint32_t *status_ptr = (uint32_t *)get_le32(&elem_status[0]);
+                LL_PWR_EnableBkUpAccess(); // In case status_ptr points to backup registers
+                *status_ptr = ret;
+            }
         }
         // Always reset because the application is expecting to resume
         led_state_all(0);

ports/stm32/mboot/mboot.h

@@ -43,6 +43,7 @@ enum {
     ELEM_TYPE_END = 1,
     ELEM_TYPE_MOUNT,
     ELEM_TYPE_FSLOAD,
+    ELEM_TYPE_STATUS,
 };
 
 enum {