toolchain-external: split target installation from staging installation

Currently, all the installation work of the toolchain-external package
is done during the install-staging step. However, in order to be able
to properly collect the size added by each package to the target
filesystem, we need to make sure that toolchain-external installs its
files to $(TARGET_DIR) during the install-target step.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: Jérôme Pouiller <jezz@sysmic.org>