stm32/mboot: Make 0x70ad0000 values named constants with macros.

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

ports/stm32/mboot/main.c

@@ -1357,7 +1357,7 @@ void stm32_main(uint32_t initial_r0) {
     }
     #endif
 
-    if ((initial_r0 & 0xffffff00) == 0x70ad0000) {
+    if ((initial_r0 & 0xffffff00) == MBOOT_INITIAL_R0_KEY) {
         goto enter_bootloader;
     }
 
@@ -1401,7 +1401,7 @@ enter_bootloader:
     mboot_pack_init();
     #endif
 
-    if ((initial_r0 & 0xffffff80) == 0x70ad0080) {
+    if ((initial_r0 & 0xffffff80) == MBOOT_INITIAL_R0_KEY_FSLOAD) {
         mboot_state_change(MBOOT_STATE_FSLOAD_START, 0);
         int ret = -1;
         #if MBOOT_FSLOAD

ports/stm32/mboot/mboot.h

@@ -46,6 +46,10 @@
 #define MBOOT_ADDRESS_SPACE_64BIT (0)
 #endif
 
+// These values are used in initial_r0 to enter mboot programatically.
+#define MBOOT_INITIAL_R0_KEY (0x70ad0000)
+#define MBOOT_INITIAL_R0_KEY_FSLOAD (MBOOT_INITIAL_R0_KEY | 0x80)
+
 // These are for led_state_all() and can be or'd together.
 #define MBOOT_LED_STATE_LED0 (0x01)
 #define MBOOT_LED_STATE_LED1 (0x02)