[package] base-files: utilize uci_get_state to read the previous dhcp router address...

[package] base-files: utilize uci_get_state to read the previous dhcp router address in udhcpc default script.
The setup_switch() procedure provided by swconfig calls config_load() during network restart and thus confuses
udhcpc's uci state keeping resulting in a missing default route on each second network restart. Bypass the
cached state vars and query uci directly to make the script immune against unwanted runtime changes.

SVN-Revision: 23211