package/mrouted: menuconfig support for enabling rsrr support

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>
[yann.morin.1998@free.fr:
  - split long lines in help text
  - use traditional ifeq-else-endif block
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>