Move bootsim-specific assert behavior behind mcuboot_config.h

Continue removing platform-specific conditional compilation from
bootutil by adding a new MCUBOOT_HAVE_ASSERT_H configuration option
and associated header file. Right now, that's only used by the
simulator.

That leaves just bootutil_log.h with platform-specific contents, but
since it's meant to be an abstraction layer for logging, we'll let it
stand for now.

Signed-off-by: Marti Bolivar <marti@opensourcefoundries.com>
diff --git a/boot/bootutil/src/bootutil_priv.h b/boot/bootutil/src/bootutil_priv.h
index b0dab08..d2ee6fc 100644
--- a/boot/bootutil/src/bootutil_priv.h
+++ b/boot/bootutil/src/bootutil_priv.h
@@ -29,8 +29,8 @@
 extern "C" {
 #endif
 
-#ifdef __BOOTSIM__
-#include "bootsim.h"
+#ifdef MCUBOOT_HAVE_ASSERT_H
+#include "mcuboot_config/mcuboot_assert.h"
 #else
 #define ASSERT assert
 #endif