package/wpa_supplicant: Allow mesh configuration

wpa_supplicant defconfig doesn't enable CONFIG_MESH
by default, and currently there is only a path to
disable it in buildroot. This patch will restore
the logic to enable it.

Signed-off-by: José Pekkarinen <jose.pekkarinen@unikie.com>
Tested-by: Nicolas Cavallari <nicolas.cavallari@green-communications.fr>
Acked-by: Nicolas Cavallari <nicolas.cavallari@green-communications.fr>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>