Move HEAP_END and RAM_END to linker script

* Issue #232

stm/gccollect.c

@@ -13,10 +13,10 @@ void gc_helper_get_regs_and_clean_stack(machine_uint_t *regs, machine_uint_t hea
 void gc_collect(void) {
     uint32_t start = sys_tick_counter;
     gc_collect_start();
-    gc_collect_root((void**)&_ram_start, ((uint32_t)&_heap_start - (uint32_t)&_ram_start) / 4);
+    gc_collect_root((void**)&_ram_start, (&_heap_start - &_ram_start) / 4);
     machine_uint_t regs[10];
-    gc_helper_get_regs_and_clean_stack(regs, HEAP_END);
-    gc_collect_root((void**)HEAP_END, (RAM_END - HEAP_END) / 4); // will trace regs since they now live in this function on the stack
+    gc_helper_get_regs_and_clean_stack(regs, (machine_uint_t)&_heap_end);
+    gc_collect_root((void**)&_heap_end, (&_ram_end - &_heap_end) / 4); // will trace regs since they now live in this function on the stack
     gc_collect_end();
     uint32_t ticks = sys_tick_counter - start; // TODO implement a function that does this properly
 

stm/gccollect.h

-#define HEAP_END  (0x2001c000) // tunable
-#define RAM_END   (0x20020000) // fixed for chip
-
 extern uint32_t _ram_start;
 extern uint32_t _heap_start;
+extern uint32_t _ram_end;
+extern uint32_t _heap_end;
 
 void gc_collect(void);
 

stm/main.c

@@ -594,7 +594,7 @@ int main(void) {
 soft_reset:
 
     // GC init
-    gc_init(&_heap_start, (void*)HEAP_END);
+    gc_init(&_heap_start, &_heap_end);
 
     // Micro Python init
     qstr_init();

stm/stm32f405.ld

@@ -19,6 +19,9 @@ _minimum_heap_size = 16K;
 /* top end of the stack */
 _estack = ORIGIN(RAM) + LENGTH(RAM);
 
+_ram_end = 0x20020000;
+_heap_end = 0x2001c000;
+
 /* define output sections */
 SECTIONS
 {