kresd: support local config file

[Alphix]

There's currently no way to change the kresd config file without
modifying the init script (changes which will be overwritten
each time kresd is updated).

This patch adds support for local configs (similarly to how
dnsmasq allows both /etc/dnsmasq.conf and uci config).

[Alphix]

This functionality has been implemented now (albeit in a slightly different manner) in the test branch -> closing this PR.

[sairon]

Hello, thanks for the contribution and sorry for the delayed reply. You are correct, the functionality is implemented in the test branch and should be released soon. We already had the plans (and some PoC code) for it when you submitted the PR, but still sorry for not writing you sooner.