bird2: does not work with MD5 password

[guillomep]

Maintainer: @tohojo
Environment: Banana pi - R4, OpenWRT snapshot

Description:

When using bird2 with an MD5 password to connect to peer we get the following error:

Socket error: Kernel does not support TCP MD5 signatures

It is because the kernel is not build with the option CONFIG_TCP_MD5SIG.

I saw that it has been solved on FRR in the following PR: openwrt/packages#24178
I don't know if it is possible to do it the same in this repository. If yes, I can purpose the PR.