dev-manual: Added a note to the EXTERNALSRC example about the class
Added a short note after the example that shows how to set up
the EXTERNALSRC variables stating that the externalsrc class
needs to be either globally or locally inherited.
(From yocto-docs rev: db935135c0b060e06f6a63530df995d286002598)
Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>