Remove radvd Makefile and patch, change docker-router-advertiser Dockerfile template to simply install the vanilla radvd package using apt-get.
- In PR https://github.com/Azure/sonic-buildimage/pull/2795, we started building radvd from source and patching it to prevent it from erroring out when advertising an MTU of 9100 which was greater than the MTU size configured on the bridge interface (1500), which was due to a limitation in the 4.9 Linux kernel.
- Master branch is now using Linux kernel 4.19. As of 4.18, the kernel supports setting a bridge MTU to a value > 1500.
- PR https://github.com/Azure/sonic-swss/pull/1393 modified vlanmgrd to take advantage of this and now configures the MTU of bridge interfaces in SONiC to the proper size of 9100. Therefore, we no longer need to patch radvd. Since we no longer need to patch radvd, we no longer need to build it from source, so we can save build time by going back to simply installing the vanilla radvd Debian package in the router-advertiser container.
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