core/br2-external: restore compatibility with old distros

Currently, the br2-external script uses bash-4's associative arrays.

However, some oldish enterprise-class distros like RHEL5 still use
bash-3.1 which lacks associative arrays.

We restore compatibility with those oldish distros using 'eval' to
emulate associative arrays, as suggested by Arnout.

Reported-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Max Filippov <jcmvbkbc@gmail.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Tested-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>