Kernel-2.eclass recently adopted EAPI 6
Let's move all kernel ebuild packages to EAPI 6

If you are wondering what is change with EAPI 6, there are some nice reference:

https://blogs.gentoo.org/mgorny/2015/11/13/the-ultimate-guide-to-eapi-6/

and from the official documentation https://dev.gentoo.org/~ulm/pms/head/pms.html:

EAPI 6 is EAPI 5 with the following changes:
  • Bash version is 4.2, bash-version on page 78.
  • Default src_prepare no longer a no-op, src-prepare-6 on page 138.
  • Different src_install implementation, src-install-6 on page 174.
  • LC_CTYPE and LC_COLLATE compatible with POSIX locale, locale-settings on page 215.
  • failglob is enabled in global scope, failglob on page 226.
  • einstall banned, banned-commands on page 233.
  • die and assert called with -n respect nonfatal, nonfatal-die on page 237.
  • eapply support, eapply on page 240.
  • eapply_user support, eapply-user on page 240.
  • econf adds --docdir and --htmldir, econf-options on page 246.
  • in_iuse support, in-iuse on page 275.
  • unpack supports absolute and relative paths, unpack-absolute on page 284.
  • unpack supports .txz, unpack-extensions on page 285.
  • unpack matches filename extensions case-insensitively, unpack-ignore-case on page 285.
  • einstalldocs support, einstalldocs on page 286.
  • get_libdir support, get-libdir on page 286.