classes/package: clarify installed-vs-shipped message

classes/package: clarify installed-vs-shipped message

Based on the number of times I've had to explain it over the years it
seems that a lot of new users don't immediately realise what is meant
by "shipped" here (nor should we expect them to) so let's at least
mention packaging and briefly tell the user what they need to do. (I was
going to go into more detail in the message, but there is really more
detail than can be covered succinctly here.)

(From OE-Core rev: 7047d02619547401c313de8c6972a7a0af77e2bc)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>