Convert BOOT_MAGIC_SZ to #define

Convert this `extern const uint32_t` to a simple define.  Provide an
accessor function so that the simulator is able to access the value as
well.  This has a minor improvement on the generated code within MCUboot
itself.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/sim/mcuboot-sys/csupport/run.c b/sim/mcuboot-sys/csupport/run.c
index dd63d55..5697948 100644
--- a/sim/mcuboot-sys/csupport/run.c
+++ b/sim/mcuboot-sys/csupport/run.c
@@ -454,3 +454,8 @@
 {
     return BOOT_MAX_ALIGN;
 }
+
+uint32_t boot_magic_sz(void)
+{
+    return BOOT_MAGIC_SZ;
+}