bootutil: Refactor MCUBOOT_LOG_MODULE_(DECLARE|REGISTER) macros

The "MCUBOOT_LOG_MODULE_(DECLARE|REGISTER)" macros should have a name
coherent with the rest of the logs macors, ie "BOOT_LOG_*". Also,
"bootutil_log.h" should define them as empty when the logs are disabled and
as "MCUBOOT_LOG_MODULE_*" when they are not.

With this change, the mcuboot user doesn't have to define
MCUBOOT_LOG_MODULE_* macros if the logs don't going to be used.

Signed-off-by: Carlos Falgueras García <carlos.falgueras@wslw.es>
diff --git a/boot/bootutil/src/loader.c b/boot/bootutil/src/loader.c
index b0fd0c7..28cb65e 100644
--- a/boot/bootutil/src/loader.c
+++ b/boot/bootutil/src/loader.c
@@ -52,7 +52,7 @@
 
 #include "mcuboot_config/mcuboot_config.h"
 
-MCUBOOT_LOG_MODULE_DECLARE(mcuboot);
+BOOT_LOG_MODULE_DECLARE(mcuboot);
 
 static struct boot_loader_state boot_data;