configure: fix --sysconfdir specification

--sysconfdir requires a parameter (the path), this should be reflected
in the case pattern.
Reported-by: NFrank Arnold <frank.arnold@amd.com>
Signed-off-by: NAndre Przywara <andre.przywara@amd.com>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>

configure

@@ -495,7 +495,7 @@ for opt do
     static="yes"
     LDFLAGS="-static $LDFLAGS"
   ;;
-  --sysconfdir) sysconfdir="$optarg"
+  --sysconfdir=*) sysconfdir="$optarg"
   ;;
   --disable-sdl) sdl="no"
   ;;