package/*: do not override LICENSE, but append to it

package/*: do not override LICENSE, but append to it

Now that the infra takes care of removing extraneous spaces before
commas, we can safely append to LICENSE variables.

This removes the only case where we had to use immediate assignment
to append to a variable.

Mechanical change, with:

    $ sed -r -i -e 's/\<([^[:space:]]+_LICENSE) := \$\(\1\),/\1 += ,/' \
        $(git grep -l -E '\<([^[:space:]]+_LICENSE) := \$\(\1\),')

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>