kernel: collect module symvers for external modules

kernel: collect module symvers for external modules

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

Example:

Before:

root@LEDE:/# modinfo mt76
module:         /lib/modules/4.4.74/mt76.ko
license:        Dual BSD/GPL
depends:

After:

root@LEDE:/# modinfo mt76
module:         /lib/modules/4.4.74/mt76.ko
license:        Dual BSD/GPL
depends:        mac80211,cfg80211

Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>