toolchain: post-pone evaluation of TOOLCHAIN_EXTERNAL_BIN

The upcoming per-package SDK functionality is heavily based on the
fact that HOST_DIR, STAGING_DIR and TARGET_DIR are evaluated during
the configure/build/install steps of the packages. Therefore, any
evaluation-during-assignment using := is going to cause problems, and
need to be turned into evaluation-during-use using =.

This patch fix up one such instance in the external toolchain code.

This change is independent from the per-package SDK functionality, and
could be applied separately.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>