kern: Align the declaration of kernconfstring with its definition

It is defined as const char[] in config.c which is auto generated by
usr.sbin/config/kernconf.tmpl .

While here prefer macro SYSCTL_CONST_STRING to avoid casting.

MFC after: 1 week

(cherry picked from commit 0f64fc6a3486454ad708f517633f930e611fd6d2)