onion_buildenv update_sdk doesn't respect default feed changes

[crispyoz]

When you run bash onion_buildenv update_sdk the function update_package_feeds creates openwrt-sdk/feeds.conf.default from a) ENV PACKAGE_FEEDS from profile then b) appending openwrt-sdk/feeds.conf.default.org.

The upshot is that the user needs to edit openwrt-sdk/feeds.conf.default.org to remove any packages you don't want to build. For instance Onion has never built luci packages, they have always been commented out the the feeds. Presently if even if you comment out the package feed in openwrt-sdk/feeds.conf.default if will overwritten by update_sdk.

I submitted PR #29 that enables the user to prefix any feed in PACKAGE_FEEDS with "-" symbol to disable the feed and the disable will survive update_sdk.

[greenbreakfast]

@crispyoz I mentioned this in the PR OnionIoT/openwrt-imagebuilder-wrapper#29, but it bears repeating here. If the packages from the default repos are not selected for compilation, then including those repos shouldn't be an issue.
Unless there's something I'm missing?