When building LittlevGL with a rather recent compiler (tested with GCC
7.3.1 provided by Arm), `-Wall`, `-Wextra` and `-Werror` activated and
`LV_LOG_LEVEL` set to `LV_LOG_LEVEL_TRACE`, compilation fails due to a
type limits error in lv_log.c
if(level >= LV_LOG_LEVEL) {
^~
because the comparison is always true, since level is a `lv_log_level_t`
(that is a typedef for `uint8_t`) and `LV_LOG_LEVEL_TRACE` equals 0.
Fix that by making `lv_log_level_t` a typedef to `int8_t`.
Fixes#690
