Makefile: enable generating documents from br2-external

Currently, the only way br2-external may generate a document is by
including that document's recipe from within external.mk.

But external.mk is only parsed when the tree is configured.

This is unlike our internal document (the manual) which can be generated
from within an unconfigured tree.

So, include the documents from br2-external at the same time we include
our own document:
  - expect the same layout as we have:      docs/DOC_NAME/doc-name.mk
  - do not fail if there is no document:    use "-include", not "include"

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Samuel Martin <s.martin49@gmail.com>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Reviewed-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>