Output boot code warning when zpool upgrade -a is used to add features.

In the case where new features where enabled by a zpool upgrade -a the
boot code warning wasn't output.

Submitted by:	Jan Kokemueller
MFC after:	3 days


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271934 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f