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/zephyr/main.c b/boot/zephyr/main.c
index a4f87e9..3664a8b 100644
--- a/boot/zephyr/main.c
+++ b/boot/zephyr/main.c
@@ -105,7 +105,7 @@
 }
 #endif
 
-MCUBOOT_LOG_MODULE_REGISTER(mcuboot);
+BOOT_LOG_MODULE_REGISTER(mcuboot);
 
 #ifdef CONFIG_MCUBOOT_INDICATION_LED
 /*