Rename BR2_PACKAGE_NETD_FRR to BR2_PACKAGE_NETD_FRR_GRPC to match the
naming convention of the other FRR backends (VTYSH, CONF) and make the
intent self-evident.
Have the gRPC backend select BR2_PACKAGE_FRR_GRPC so FRR is built with
gRPC support when — and only when — this backend is chosen.
Signed-off-by: Joachim Wiberg <troglobit@gmail.com>