stm32/mboot: Fix mp_hal_delay_us() and add mp_hal_ticks_ms().

Signed-off-by: Damien George <damien@micropython.org>

ports/stm32/mboot/main.c

@@ -95,6 +95,9 @@
 // These bits are used to detect valid application firmware at APPLICATION_ADDR
 #define APP_VALIDITY_BITS (0x00000003)
 
+// For 1ms system ticker.
+static volatile uint32_t systick_ms;
+
 // Global dfu state
 dfu_context_t dfu_context SECTION_NOZERO_BSS;
 
@@ -104,6 +107,10 @@ uint32_t get_le32(const uint8_t *b) {
     return b[0] | b[1] << 8 | b[2] << 16 | b[3] << 24;
 }
 
+mp_uint_t mp_hal_ticks_ms(void) {
+    return systick_ms;
+}
+
 void mp_hal_delay_us(mp_uint_t usec) {
     // use a busy loop for the delay
     // sys freq is always a multiple of 2MHz, so division here won't lose precision
@@ -113,11 +120,10 @@ void mp_hal_delay_us(mp_uint_t usec) {
     const uint32_t ucount = SystemCoreClock / 2000000 * usec / 2;
     #endif
     for (uint32_t count = 0; ++count <= ucount;) {
+        __NOP();
     }
 }
 
-static volatile uint32_t systick_ms;
-
 void mp_hal_delay_ms(mp_uint_t ms) {
     if (__get_PRIMASK() == 0) {
         // IRQs enabled, use systick