merge_config.sh: add br2-external support

Pass BR2_EXTERNAL value via -e option.
This will prevent merge_config.sh from silently eating any symbols defined in
external trees on a clean buildroot tree invocation.

Signed-off-by: Ilya Kuzmich <ilya.kuzmich@gmail.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>