rtmpdump: new package

rtmpdump - RTMPDump Real-Time Messaging Protocol API
This package was originally found at : https://github.com/huceke/buildroot-rbp


By gimli <ebsi4711@gmail.com>

Note that this package will only install librtmp in this state.
Hence the name shown librtmp instead of rtmpdump

[Peter: whitespace fixes, use RTMPDUMP_CFLAGS]
Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Cc: gimli <ebsi4711@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>