core/br2-external: export paths and descriptions

core/br2-external: export paths and descriptions

Now that we support multiple br2-external trees, BR2_EXTERNAL is no
longer exported in the environment.

This means that post-build scripts in a br2-external tree can no longer
find their own files (well, they could re-invent the path by stripping
their known-relative path, but that'd be ugly, especially since we can
very well provide it).

Export the path for each br2-external trees as environment variables.
Do so for the description as well, as a courtesy.

Also, re-order variable definitions to be more logical: first, purely
internal variables, then exported variables.

Reported-by: Benoît Allard <benoit.allard@greenbone.net>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Benoît Allard <benoit.allard@greenbone.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>