toolchain-external: fix Linaro 2014.02 symlink

In commit cd32da8f
("toolchain-external: add Linaro ARM big endian toolchain") a mistake
was made, probably due to a rebase conflict that was incorrectly solved:
the call to the post install staging hook that creates the necessary
symbolic links for a root filesystem based on Linaro 2014.02 to work
was removed.

This commit reinstates this call, which should fix the problem
observed by Maxime Hadjinlian while using Linaro 2014.02.

Reported-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>