phase1: add support for overriding feeds host

phase1: add support for overriding feeds host

Currently git.openwrt.org is sometimes overloaded, so lets implement a
mechanism to override the source host for feeds.

This introduces a `feeds_host_override` configuration option. When set,
the buildmaster will temporarily modify `feeds.conf.default` to point to
the alternate host (e.g., GitHub) before updating feeds, and restore the
original configuration afterwards.

This is particularly useful when the primary git server is returning 503
errors:

    Updating feed 'packages' from 'https://git.openwrt.org/feed/packages.git' ...
    Cloning into './feeds/packages'...
    fatal: unable to access 'https://git.openwrt.org/feed/packages.git/': The requested URL returned error: 503
    failed.

Signed-off-by: Petr Štetiar <[email protected]>