Corrected the read_mcast_stats function, to display
"ubus call mcast stats" correct output for multiple iptv bridges
Test:
"ubus call mcast stats" showing correct output for both single and multiple bridge.
Fixed worng temp_igmp_file variable accessing, which caused the same
'temp_igmp_file' file to be used each time 'ubus call mcast stats'
was called instead of /tmp/igmp_stats_<PID> file.
Because of this, temp file was sometimes overwritten by the next call
before it had time to read and the IPTV_IGMP_4-12-1-24_2401.py test
ended with an error.
A waiting loop has also been added so that mcproxy has time to process
the signal and write the dump to a file.
Script for generating mcast stats is moved from common to platform
specific files. The logic for generating mcast stats on non broadcom
platforms is different and the stats format is also different.
For generating stats we are are sending SIGUSR1 signal to mcproxy
process which then writes multicast stats to /tmp/igmp_snooping_stats.
