Work around slow to attach boot devices.

If your boot device takes a bit to attach you can end up at the
FreeBSD mountroot> prompt.  If you fiddlefuck with that too many
times the system will panic.  Since we have a bit of a heart and don't
expect mere mortals (or users) to know the FreeBSD mountroot syntax
or that they should wait a while to see if their boot device magically
appears before typing arcane things like zfs:freenas-boot let's add a
30 second delay to see if the boot device appears before bailing out and
ending up at the mountroot prompt.  This delay should satisfy startup
requirements for the boot devices even CyberJock's Mom uses, and if
you know CyberJock's Mom you'll know just how cheap she is.

Ticket: #7254
(cherry picked from commit 2cd714a95c596b3ab88aa984e192a5ad14c6abca)