kernel: enhance multicast routing support

kernel: enhance multicast routing support

Certain utilities, such as smcroute [1], require additional multicast
routing options to be enabled, otherwise they will not function
correctly. Enable these relevant dependancies when IPv4 and/or IPv6
multicast routing are enabled.

[1] https://github.com/troglobit/smcroute/blob/master/README.md#linux-requirements

This increases the uncompressed kernel size on MIPS 24kc by 8KBytes
and the compressed kernel size by 1.8KBytes.

Signed-off-by: Matthew Hagan <mnhagan88@gmail.com>