My command:

pi@raspberrypi:~/ffmpeg $ ./ffmpeg -f x11grab -framerate 1 -thread_queue_size 512 -i :0.0 -f lavfi -i anullsrc -c:v h264_omx -b:v 4096k -probesize 50M -g 60 -c:a aac -b:a 128k -f flv rtmp://a.rtmp.youtube.com/live2/xxxxxx


My output from ffmpeg:

ffmpeg version N-90073-gd2fc244293 Copyright (c) 2000-2018 the FFmpeg developers
  built with gcc 6.3.0 (Raspbian 6.3.0-18+rpi1) 20170516
  configuration: --arch=armel --target-os=linux --enable-gpl --enable-omx --enable-omx-rpi --enable-nonfree --enable-mmal
  libavutil      56.  7.101 / 56.  7.101
  libavcodec     58. 11.101 / 58. 11.101
  libavformat    58.  9.100 / 58.  9.100
  libavdevice    58.  1.100 / 58.  1.100
  libavfilter     7. 12.100 /  7. 12.100
  libswscale      5.  0.101 /  5.  0.101
  libswresample   3.  0.101 /  3.  0.101
  libpostproc    55.  0.100 / 55.  0.100
[x11grab @ 0x22e7480] Stream #0: not enough frames to estimate rate; consider increasing probesize
Input #0, x11grab, from ':0.0':
  Duration: N/A, bitrate: N/A
    Stream #0:0: Video: rawvideo (BGR[0] / 0x524742), bgr0, 640x480, 1 fps, 1000k tbr, 1000k tbn, 1000k tbc
Input #1, lavfi, from 'anullsrc':
  Duration: N/A, start: 0.000000, bitrate: 705 kb/s
    Stream #1:0: Audio: pcm_u8, 44100 Hz, stereo, u8, 705 kb/s
Stream mapping:
  Stream #0:0 -> #0:0 (rawvideo (native) -> h264 (h264_omx))
  Stream #1:0 -> #0:1 (pcm_u8 (native) -> aac (native))
Press [q] to stop, [?] for help
[h264_omx @ 0x22f4ad0] Using OMX.broadcom.video_encode
Output #0, flv, to 'rtmp://a.rtmp.youtube.com/live2/dz9t-g0sk-wczb-3qth':
  Metadata:
    encoder         : Lavf58.9.100
    Stream #0:0: Video: h264 (h264_omx) ([7][0][0][0] / 0x0007), yuv420p(progressive), 640x480, q=2-31, 4096 kb/s, 1 fps, 1k tbn, 1 tbc
    Metadata:
      encoder         : Lavc58.11.101 h264_omx
    Stream #0:1: Audio: aac (LC) ([10][0][0][0] / 0x000A), 44100 Hz, stereo, fltp, 128 kb/s
    Metadata:
      encoder         : Lavc58.11.101 aac
frame=    5 fps=0.0 q=-0.0 size=     110kB time=00:00:03.04 bitrate= 295.1kbitframe=    5 fps=5.0 q=-0.0 size=     110kB time=00:00:03.04 bitrate= 295.1kbitframe=    5 fps=3.3 q=-0.0 size=     110kB time=00:00:03.04 bitrate= 295.1kbitframe=    5 fps=2.5 q=-0.0 size=     110kB time=00:00:03.04 bitrate= 295.1kbitframe=    5 fps=2.0 q=-0.0 size=     110kB time=00:00:03.04 bitrate= 295.1kbitframe=    5 fps=1.7 q=-0.0 size=     110kB time=00:00:03.04 bitrate= 295.1kbitframe=    5 fps=1.4 q=-0.0 size=     110kB time=00:00:03.04 bitrate= 295.1kbitframe=    5 fps=1.2 q=-0.0 size=     110kB time=00:00:03.04 bitrate= 295.1kbitframe=    5 fps=1.1 q=-0.0 size=     110kB time=00:00:03.04 bitrate= 295.1kbitframe=    5 fps=1.0 q=-0.0 size=     110kB time=00:00:03.04 bitrate= 295.1kbitframe=    5 fps=0.9 q=-0.0 size=     110kB time=00:00:03.04 bitrate= 295.1kbitframe=    5 fps=0.8 q=-0.0 size=     110kB time=00:00:03.04 bitrate= 295.1kbitframe=    5 fps=0.8 q=-0.0 size=     110kB time=00:00:03.04 bitrate= 295.1kbitframe=    5 fps=0.7 q=-0.0 size=     110kB time=00:00:03.04 bitrate= 295.1kbitframe=    5 fps=0.7 q=-0.0 size=     110kB time=00:00:03.04 bitrate= 295.1kbitframe=    5 fps=0.6 q=-0.0 size=     110kB time=00:00:03.04 bitrate= 295.1kbitframe=    5 fps=0.6 q=-0.0 size=     110kB time=00:00:03.04 bitrate= 295.1kbitframe=    5 fps=0.6 q=-0.0 size=     110kB time=00:00:03.04 bitrate= 295.1kbitframe=    5 fps=0.5 q=-0.0 size=     110kB time=00:00:03.04 bitrate= 295.1kbitframe=    5 fps=0.5 q=-0.0 size=     110kB time=00:00:03.04 bitrate= 295.1kbitframe=    5 fps=0.5 q=-0.0 size=     110kB time=00:00:03.04 bitrate= 295.1kbitframe=    5 fps=0.5 q=-0.0 size=     110kB time=00:00:03.04 bitrate= 295.1kbitframe=    5 fps=0.4 q=-0.0 size=     110kB time=00:00:03.04 bitrate= 295.1kbit