kernel: collect module symvers for external modules (#1580)

Collect module symvers for all external modules to make them available
for modpost. This fixes dependencies for most external modules.

  root@ffv-525400123456:/# modinfo batman-adv
  module:         /lib/modules/4.4.153/batman-adv.ko
  alias:          net-pf-16-proto-16-family-batadv
  alias:          rtnl-link-batadv
  version:        openwrt-2018.1-5
  description:    B.A.T.M.A.N. advanced
  author:         Marek Lindner <mareklindner@neomailbox.ch>, Simon Wunderlich <sw@simonwunderlich.de>
  license:        GPL
  depends:

After:

  root@ffv-525400123456:/# modinfo batman-adv
  module:         /lib/modules/4.4.153/batman-adv.ko
  alias:          net-pf-16-proto-16-family-batadv
  alias:          rtnl-link-batadv
  version:        openwrt-2018.1-5
  description:    B.A.T.M.A.N. advanced
  author:         Marek Lindner <mareklindner@neomailbox.ch>, Simon Wunderlich <sw@simonwunderlich.de>
  license:        GPL
  depends:        libcrc32c,cfg80211