sim: log: add new level targetting simulator

* Adds a new level (BOOT_LOG_SIM) to be used only for messages that
  are interesting while debugging bootutil in the simulator. This should
  be used for extra verbose prints.

* Also added fflushs after fprints to guarantee that messages are printed
  even when assertions are raised.

* For abstraction completeness, add "do nothing" definitions of _LOG_SIM
  to the other ports.

* Make DEBUG the default level when building the simulator (one can
  still lower verbosity using any other value for RUST_LOG).

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/sim/mcuboot-sys/src/api.rs b/sim/mcuboot-sys/src/api.rs
index f38d98e..a474e8f 100644
--- a/sim/mcuboot-sys/src/api.rs
+++ b/sim/mcuboot-sys/src/api.rs
@@ -209,7 +209,8 @@
         1 => log_enabled!(Level::Error),
         2 => log_enabled!(Level::Warn),
         3 => log_enabled!(Level::Info),
-        4 => log_enabled!(Level::Trace),
+        4 => log_enabled!(Level::Debug),
+        5 => log_enabled!(Level::Trace), // log level == SIM
         _ => false,
     };
     if res {