qemu-arm: Rework to run bare-metal on boards with Cortex-M CPUs.

Adds support for 3 Cortex-M boards, selectable via "BOARD" in the Makefile:
- microbit, Cortex-M0 via nRF51822
- netduino2, Cortex-M3 via STM32F205
- mps2-an385, Cortex-M3 via FPGA

netduino2 is the default board because it's supported by older qemu
versions (down to at least 2.5.0).

ports/qemu-arm/test_main.c

@@ -15,15 +15,14 @@
 #include "tinytest.h"
 #include "tinytest_macros.h"
 
-#define HEAP_SIZE (128 * 1024)
-STATIC void *heap;
+#define HEAP_SIZE (100 * 1024)
 
 #include "genhdr/tests.h"
 
 int main() {
     mp_stack_ctrl_init();
     mp_stack_set_limit(10240);
-    heap = malloc(HEAP_SIZE);
+    static uint32_t heap[HEAP_SIZE / sizeof(uint32_t)];
     upytest_set_heap(heap, (char*)heap + HEAP_SIZE);
     int r = tinytest_main(0, NULL, groups);
     printf("status: %d\n", r);
@@ -34,8 +33,7 @@ void gc_collect(void) {
     gc_collect_start();
 
     // get the registers and the sp
-    jmp_buf env;
-    setjmp(env);
+    // TODO get registers
     volatile mp_uint_t dummy;
     void *sp = (void*)&dummy;