package/bird: make RIP optional

package/bird: make RIP optional

In commit 529fef57f6f46201ba16349b49c0a65ceccc4c25 ("package/bird: add
optional protocols") we added options to conditionally enable
different protocol support. However, disabling the RIP support was not
working, so it remained forcefully enabled, and there was no
BR2_PACKAGE_BIRD_RIP option.

However, since the bump to 2.0.6 in commit
b9f43ade0fbce38b803214c167df52ca7fd3ee03, this issue has been fixed,
so we can introduce the BR2_PACKAGE_BIRD_RIP option, and make it
really optional.

Signed-off-by: Adrien Gallouët <adrien@gallouet.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>