NAT: move BSC config into separate file

Introduce new configuration option bscs-config-file which includes BSC
configuration from the given file. Both absolute and relative (to the
main config file) paths are supported.
Add 'show bscs-config' command to display current BSC configuration.
Note: it is still possible to have BSC configuration in the main
file (provided proper index number is used) and in runtime but BSC
configuration is no longer saved automatically. The management of
included configuration file is left to external tools.
Update configuration examples.

Fixes: OS#1669
Sponsored-by: On-Waves ehf
diff --git a/openbsc/doc/examples/osmo-bsc_nat/osmo-bsc_nat.cfg b/openbsc/doc/examples/osmo-bsc_nat/osmo-bsc_nat.cfg
index 737d104..2e00bc2 100644
--- a/openbsc/doc/examples/osmo-bsc_nat/osmo-bsc_nat.cfg
+++ b/openbsc/doc/examples/osmo-bsc_nat/osmo-bsc_nat.cfg
@@ -62,11 +62,5 @@
  timeout ping 20
  timeout pong 5
  ip-dscp 0
+ bscs-config-file bscs.config
  access-list bla imsi-allow ^11$
-
- bsc 0
-  token bla
-  location_area_code 1234
-  description bsc
-  max-endpoints 32
-  paging forbidden 0