The "feature(name)" predicate will return true if <name> corresponds to
a name listed after a '+' in the features list, that is it was enabled at
build time with USE_<name>=1. Typical use cases will include OPENSSL, LUA
and LINUX_SPLICE. But maybe it will also be convenient to use with optional
addons such as PROMEX and the device detection modules to help keeping the
same configs across various deployments.
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