Separate application and core driver logging

Introduce a new CORE_LOG_SEVERITY variable to set the log level desired
for core software/core platform applications. Separating it from the
ETHOSU_LOG_SEVERITY setting that now only affect the core_driver log
level. Add a new CORE_LOG_ENABLE variable that allows all logs
(including the unconditional LOG macro) to be turned off. Note however
that it will not affect printf and similar calls, but only logs going
through the ethosu_log library.

Sync the ethosu_log library with changes from the intended standalone
copy in core_driver.

Change-Id: I92a1254a2dd224f6403318f983c63236ea7d6aa8
Signed-off-by: Jonny Svärd <jonny.svaerd@arm.com>