package/batman-adv: fix compile with BR2_PACKAGE_BATMAN_ADV_BATMAN_V disabled

package/batman-adv: fix compile with BR2_PACKAGE_BATMAN_ADV_BATMAN_V disabled

Commit e8b1eeb2f3f5 (package/batman-adv: fix compile with
BR2_PACKAGE_BATMAN_ADV_BATMAN_V disabled) was tested against an RPi4
linux kernel already enabling the build-in batman-adv module inlcusive
batman-v, hence it missed the case where the in-tree module is not
enabled.

Taking a deeper look at the configure script gen-compat-autoconf.sh
reveals that the batman feature options must be explicitly set to 'y' or
'n' to work as expected.

Fixes:

  ERROR: modpost: "batadv_v_mesh_free" [.../build/batman-adv-2021.0/net/batman-adv/batman-adv.ko] undefined!
  ERROR: modpost: "batadv_v_mesh_init" [.../build/batman-adv-2021.0/net/batman-adv/batman-adv.ko] undefined!
  ERROR: modpost: "batadv_v_hardif_init" [.../build/batman-adv-2021.0/net/batman-adv/batman-adv.ko] undefined!
  ERROR: modpost: "batadv_v_init" [.../build/batman-adv-2021.0/net/batman-adv/batman-adv.ko] undefined!

Reported-by: Christian Stewart <christian@paral.in>
Signed-off-by: Peter Seiderer <ps.report@gmx.net>
[yann.morin.1998@free.fr: add blurb about tests on previous commit]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>