Place Memfault reboot tracking in backup SRAM

This should ensure the reboot tracking structure stays in the same
location through an OTA, where the zephyr `.noinit` region can move
(since the `.noinit` structure is placed after `.bss` / `.data`, which
can change in size from version to version). Note that an absolute
address, via DTS overlay, would be required if other data is placed into
the `BACKUP_SRAM` output section, to avoid accidentally relocating the
structure.

boards/stm32h7b3i_dk.conf

@@ -0,0 +1,5 @@
+# Enable backup SRAM for storing reboot reason
+CONFIG_STM32_BACKUP_SRAM=y
+
+# Place the reboot reason structure in the backup SRAM output section
+CONFIG_MEMFAULT_REBOOT_TRACKING_REGION="BACKUP_SRAM"

boards/stm32h7b3i_dk.overlay

@@ -11,6 +11,12 @@
 	status = "okay";
 };
 
+/* Enable backup SRAM to store reboot reason through OTA- the location of the
+reboot reason structure should be fixed from version to version */
+&backup_sram {
+	status = "okay";
+};
+
 &adc2 {
 	pinctrl-0 = <&adc2_inp0_pc2_c>;
 	pinctrl-names = "default";

config/memfault_platform_config.h

@@ -10,11 +10,6 @@
 extern "C" {
 #endif
 
-#if defined(CONFIG_SOC_FAMILY_ESP32)
-  #define ZEPHYR_DATA_REGION_START _data_start
-  #define ZEPHYR_DATA_REGION_END _data_end
-#endif
-
 #ifdef __cplusplus
 }
 #endif