bgpd: Move sticky, default_gw, router_flag into a single flags variable

bgpd: Move sticky, default_gw, router_flag into a single flags variable

Instead of using 3 uint8_t variables under struct attr, let's use a single
uint8_t as the flags. Saving 2-bytes. Not a big deal, but it's even easier to
track EVPN-related flags/variables.

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>