stm32/main: Add configuration macros for board to set heap start/end.

The macros are MICROPY_HEAP_START and MICROPY_HEAP_END, and if not defined
by a board then the default values will be used (maximum heap from SRAM as
defined by linker symbols).

As part of this commit the SDRAM initialisation is moved to much earlier in
main() to potentially make it available to other peripherals and avoid
re-initialisation on soft-reboot.  On boards with SDRAM enabled the heap
has been set to use that.

ports/stm32/boards/STM32F429DISC/mpconfigboard.h

@@ -76,6 +76,8 @@
 // SDRAM
 #define MICROPY_HW_SDRAM_SIZE  (64 / 8 * 1024 * 1024)  // 64 Mbit
 #define MICROPY_HW_SDRAM_STARTUP_TEST             (1)
+#define MICROPY_HEAP_START              sdram_start()
+#define MICROPY_HEAP_END                sdram_end()
 
 // Timing configuration for 90 Mhz (11.90ns) of SD clock frequency (180Mhz/2)
 #define MICROPY_HW_SDRAM_TIMING_TMRD        (2)

ports/stm32/boards/STM32F769DISC/mpconfigboard.h

@@ -83,6 +83,8 @@
 // Optional SDRAM configuration; requires SYSCLK <= 200MHz
 #define MICROPY_HW_SDRAM_SIZE (128 * 1024 * 1024 / 8) // 128 Mbit
 #define MICROPY_HW_SDRAM_STARTUP_TEST (0)
+#define MICROPY_HEAP_START sdram_start()
+#define MICROPY_HEAP_END sdram_end()
 
 // Timing configuration for 90 Mhz (11.90ns) of SD clock frequency (180Mhz/2)
 #define MICROPY_HW_SDRAM_TIMING_TMRD        (2)

ports/stm32/boards/STM32F7DISC/mpconfigboard.h

@@ -84,6 +84,8 @@ void STM32F7DISC_board_early_init(void);
 // SDRAM
 #define MICROPY_HW_SDRAM_SIZE  (64 / 8 * 1024 * 1024)  // 64 Mbit
 #define MICROPY_HW_SDRAM_STARTUP_TEST             (1)
+#define MICROPY_HEAP_START              sdram_start()
+#define MICROPY_HEAP_END                sdram_end()
 
 // Timing configuration for 90 Mhz (11.90ns) of SD clock frequency (180Mhz/2)
 #define MICROPY_HW_SDRAM_TIMING_TMRD        (2)

ports/stm32/main.c

@@ -498,6 +498,12 @@ void stm32_main(uint32_t reset_mode) {
     #endif
 
     // basic sub-system init
+    #if MICROPY_HW_SDRAM_SIZE
+    sdram_init();
+    #if MICROPY_HW_SDRAM_STARTUP_TEST
+    sdram_test(true);
+    #endif
+    #endif
     #if MICROPY_PY_THREAD
     pyb_thread_init(&pyb_thread_main);
     #endif
@@ -556,16 +562,7 @@ soft_reset:
     mp_stack_set_limit((char*)&_estack - (char*)&_heap_end - 1024);
 
     // GC init
-    #if MICROPY_HW_SDRAM_SIZE
-    sdram_init();
-    #if MICROPY_HW_SDRAM_STARTUP_TEST
-    sdram_test(true);
-    #endif
-
-    gc_init(sdram_start(), sdram_end());
-    #else
-    gc_init(&_heap_start, &_heap_end);
-    #endif
+    gc_init(MICROPY_HEAP_START, MICROPY_HEAP_END);
 
     #if MICROPY_ENABLE_PYSTACK
     static mp_obj_t pystack[384];

ports/stm32/mpconfigboard_common.h

@@ -115,6 +115,14 @@
 /*****************************************************************************/
 // General configuration
 
+// Heap start / end definitions
+#ifndef MICROPY_HEAP_START
+#define MICROPY_HEAP_START &_heap_start
+#endif
+#ifndef MICROPY_HEAP_END
+#define MICROPY_HEAP_END &_heap_end
+#endif
+
 // Configuration for STM32F0 series
 #if defined(STM32F0)