Update full-openwrt-build.md

[wifinigel]

[simonmorley]

@wifinigel Looks good generally. :)

Could I request you broke the massive code block up into smaller pieces. Maybe at each new command, not sure what would work,

[Helias]

In the line

echo "src-git cucumber http://github.com/cucumber-tony/cucumber-feeds.git;for-15.05" >> feeds.conf.default

Remove ;for-15.05 that branch doesn't exist more, so just remove it to use master as default.