toolchain-external: allow specifying extra external libraries

Custom toolchains may provide extra libraries that need to be copied to the
target. This patch adds a configuration option for this.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>