toolchain-external: fix rebuild/reinstall for Linaro toolchains

For Linaro toolchains, a special post install staging hook is used to
create two symlinks needed for the dynamic loader to find the
libraries. However, the way the link is created prevents a 'make
toolchain-external-reinstall' from succeeding, because the symlink
already exists and points to a directory:

ln -sf . /home/thomas/projets/outputs/training/target/lib/arm-linux-gnueabihf
ln: '/home/thomas/projets/outputs/training/target/lib/arm-linux-gnueabihf/.': cannot overwrite directory

This commit adjust the hook to pass the '-n' option so that the link
name is treated as a normal file if it is a symbolic link to a
directory.

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