krb5: add native and nativesdk extend

add native and nativesdk extend, curl-native/nativesdk need them.
replace the hardcode /etc with ${sysconfdir}, /var with ${localstatedir}

Signed-off-by: Roy Li <rongqing.li@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Armin Kuster <akuster808@gmail.com>