log: handle LOG_*() before calling to log_init()

log: handle LOG_*() before calling to log_init()

There are cases where LOG_*() functions are called before the logs
are initialized with log_init().
E.g. in transport_register() that is executed in gcc constructors,
thus called even before main().
With debug_level set to LOG_LVL_USER=-1 all the LOG_ERROR() get
dropped.
Properly initializing debug_level cause segmentation fault due to
log_output still not initialized.

Initialize debug_level to LOG_LVL_INFO so errors get printed.
Handle separately the case of log_output still NULL, meaning that
log_init() is not called yet.

Change-Id: I2ea32c87a4955fb44e79b38131c456e25dfbc327
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5602
Tested-by: jenkins