manual: switch BR2_EXTERNAL examples to use absolute paths

Although it is possible to use relative paths, there are a few pitfalls
when doing so.

To avoid confusion for a (newcoming) user, use absolute paths in the
manual (as is done in examples for $(O)), since it is guaranteed to be
working without corner cases.

[Peter: s/relatively/relative/ as suggested by Thomas]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>