support/scripts/br2-external: declare missing local variables

support/scripts/br2-external: declare missing local variables

Commit b14b02698 (core/br2-external: restore compatibility with old
distros) switched to using 'eval' to emulate associative arrays, for
those distros too old to have bash-4+.

In so doing, it forgot to declare the new local variables in the
respective helper functions.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Vadim Kochan <vadim4j@gmail.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>