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/flash_map_legacy.c b/boot/zephyr/flash_map_legacy.c
index b217947..6a8e7ae 100644
--- a/boot/zephyr/flash_map_legacy.c
+++ b/boot/zephyr/flash_map_legacy.c
@@ -49,7 +49,7 @@
 #define FLASH_AREA_IMAGE_SECTOR_SIZE FLASH_AREA_IMAGE_SCRATCH_SIZE
 #endif
 
-MCUBOOT_LOG_MODULE_DECLARE(mcuboot);
+BOOT_LOG_MODULE_DECLARE(mcuboot);
 
 /*
  * Lookup the sector map for a given flash area.  This should fill in