kconfig: use true and false for bool variable

fixed the following coccicheck:
./scripts/kconfig/confdata.c:36:9-10: WARNING: return of 0/1 in function
'is_dir' with return type bool

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>