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>