allow pi to use ffmpeg

@@ -190,7 +190,7 @@ if [ $SRS_PI = YES ]; then
     SRS_HLS=YES
     SRS_NGINX=NO
     SRS_SSL=YES
-    SRS_FFMPEG=NO
+    if [ $SRS_FFMPEG = RESERVED ]; then SRS_FFMPEG=NO; fi
     SRS_HTTP_CALLBACK=NO
     SRS_HTTP_SERVER=YES
     SRS_HTTP_API=YES