buildscript: bump up OpenWRT

New version is c75367d20240da182fcbdb68c8cce64eabd71ac5
- Security fixes (glibc)
- Kernel update 3.18.27
- ar71xx: ethernet fixes
- mac80211: new version
- support WR741v5
- odhcp6c: minor fixes
- support WR741v5, WR1043v3, WR941v6

Signed-off-by: Tim Niemeyer <tim@tn-x.org>
Reviewed-by: Tobias Klaus <tk+ff@meskal.net>