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/sim/mcuboot-sys/csupport/bootsim.h b/sim/mcuboot-sys/csupport/bootsim.h
index f3fc5e4..3245ec3 100644
--- a/sim/mcuboot-sys/csupport/bootsim.h
+++ b/sim/mcuboot-sys/csupport/bootsim.h
@@ -1,7 +1,6 @@
 #ifndef H_BOOTSIM_
 #define H_BOOTSIM_
 
-void sim_assert(int, const char *test, const char *, unsigned int, const char *);
-#define ASSERT(x) sim_assert((x), #x, __FILE__, __LINE__, __func__)
+#include "mcuboot_config/mcuboot_assert.h"
 
 #endif