toolchain-external: make extraction idempotent

Commit 23ffa7ec first extracts to the toolchain-external build
directory and then moves everything to $(HOST_DIR)/opt/ext-toolchain.
However, this is not idempotent, because moving directories over
existing ones doesn't always work, particularly if the target is on
another device.

Simply remove the destination contents before moving.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>