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>

toolchain/toolchain-external/toolchain-external.mk

@@ -483,6 +483,7 @@ TOOLCHAIN_EXTERNAL_EXCLUDES = usr/lib/locale/*
 # since it's actually a fairly special package, we need it to be moved
 # into TOOLCHAIN_EXTERNAL_INSTALL_DIR.
 define TOOLCHAIN_EXTERNAL_MOVE
+	rm -rf $(TOOLCHAIN_EXTERNAL_INSTALL_DIR)/*
 	mkdir -p $(TOOLCHAIN_EXTERNAL_INSTALL_DIR)
 	mv $(@D)/* $(TOOLCHAIN_EXTERNAL_INSTALL_DIR)/
 endef