implement _sbrk_r syscall

Without implementing _sbrk_r, the weak symbol from newlib-nano is used which calls _sbrk which does not check for RAM end:
```
_sbrk:
ldr     r2, [pc, #16]   ; (8013fa0 <_sbrk+0x14>)
ldr     r1, [pc, #20]   ; (8013fa4 <_sbrk+0x18>)
ldr     r3, [r2, #0]
cmp     r3, #0
it      eq
moveq   r3, r1
add     r0, r3
str     r0, [r2, #0]
mov     r0, r3
bx      lr
```

```
void** heap_end;
void* end;

void* _sbrk(size_t size) {
    void* r3 = *heap_end;
    if (r3 == NULL)
        r3 = end;
    size += r3;
    *heap_end = size;
    return r3;
}
```

The implementation in this commit returns an error code if the heap would otherwise extend into the reserved stack memory of 0x800 bytes.

Firmware/Board/v3.3/STM32F405RGTx_FLASH.ld

@@ -37,6 +37,7 @@ _estack = 0x20020000;    /* end of RAM */
 /* Generate a link error if heap and stack don't fit into RAM */
 _Min_Heap_Size = 0x3C00;      /* required amount of heap  */
 _Min_Stack_Size = 0x800; /* required amount of stack */
+_heap_end_max = _estack - _Min_Stack_Size;
 
 /* Specify the memory areas */
 MEMORY

Firmware/Board/v3.3/Src/syscalls.c

@@ -1,7 +1,7 @@
 /**
   ******************************************************************************
   * @file           : syscalls.c
-  * @brief          : This file implements printf functionality
+  * @brief          : This file implements OS functionality needed by newlib
   ******************************************************************************
 */
 
@@ -19,6 +19,45 @@
 //int _fstat(int file, struct stat *st) {}
 //int _isatty(int file) {}
 
+extern char _end; // provided by the linker script
+extern char _heap_end_max; // provided by the linker script
+void* _end_ptr = &_end;
+void* _heap_end_max_ptr = &_heap_end_max;
+void* heap_end_ptr = 0;
+
+/* @brief Increments the program break (aka heap end)
+*
+* This is called internally by malloc once it runs out
+* of heap space. Malloc might expect a contiguous heap,
+* so we don't call the FreeRTOS pvPortMalloc here.
+* If this function returns -1, malloc will return NULL.
+* Note that if this function returns NULL, malloc does not
+* consider this as an error and will return the pointer 0x8.
+*
+* You should still be careful with using malloc though,
+* as it does not guarantee thread safety.
+*
+* @return A pointer to the newly allocated block on success
+*         or -1 otherwise.
+*/
+intptr_t _sbrk_r(struct _reent *ignored __attribute__((unused)), size_t size) {
+    intptr_t ptr;
+	vTaskSuspendAll();
+	{
+        if (!heap_end_ptr)
+            heap_end_ptr = _end_ptr;
+        if (heap_end_ptr + size > _heap_end_max_ptr) {
+            ptr = -1;
+        } else {
+            ptr = (intptr_t)heap_end_ptr;
+            heap_end_ptr += size;
+        }
+	}
+	(void)xTaskResumeAll();
+    return ptr;
+}
+
+
 #define UART_TX_BUFFER_SIZE 64
 static uint8_t uart_tx_buf[UART_TX_BUFFER_SIZE];