ebuilds set a bunch of top-level variables without necessarily using them (they are consumed externally through portage).  This results in tons of warnings like these:

In shellcheck-0.5.0-r3.ebuild line 4:
EAPI=6
^-- SC2034: EAPI appears unused. Verify use (or export if used externally).

We currently disable SC2034 in cros lint, but it's generally a useful warning.  We could turn it back on if we add a list of known top-level ebuild variables that shouldn't be warned about.  A good starting point might be the list at https://devmanual.gentoo.org/ebuild-writing/variables/index.html