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/samples/mcuboot_config/mcuboot_config.template.h b/samples/mcuboot_config/mcuboot_config.template.h
index 75ec500..b9997b0 100644
--- a/samples/mcuboot_config/mcuboot_config.template.h
+++ b/samples/mcuboot_config/mcuboot_config.template.h
@@ -82,4 +82,13 @@
  * as desirable. */
 #define MCUBOOT_MAX_IMG_SECTORS 128
 
+/*
+ * Assertions
+ */
+
+/* Uncomment if your platform has its own mcuboot_config/mcuboot_assert.h.
+ * If so, it must provide an ASSERT macro for use by bootutil. Otherwise,
+ * "assert" is used. */
+/* #define MCUBOOT_HAVE_ASSERT_H */
+
 #endif /* __MCUBOOT_CONFIG_H__ */