toolchain/toolchain-external: install ldd on the target

From: Julien Boibessot <julien.boibessot@armadeus.com>

It could be usefull to have ldd on the target so install it.

Signed-off-by: Julien Boibessot <julien.boibessot@armadeus.com>
[Sébastien: add commit message]
Signed-off-by: Sébastien Szymanski <sebastien.szymanski@armadeus.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>