Hello,
You have glibc 2.19 < 2.20, generally then _old_ should be used, but in this case also package without _old_ should works.
So, I suggest you to install mipsel fpu_soft first and check if ffmpeg is working:
ffmpeg -version
If you mean flv12mpeg4 converted it is included in exteplayer3 not in ffmpeg libs.
At now it is in test phase.
Regards,
SSS