core/legal-info: don't require overriding _LICENSE

core/legal-info: don't require overriding _LICENSE

Currently, the formatting we impose on the _LICENSE variable requires
that we also use the rarely used := assignment operator, which makes
the _LICENSE variable the only variable that users have to write with
this operator.

This really departs from the simplicity and consistency of using the
append-assignment, which we use for every other variable.

This is because the append-assignment operator surreptiously
introduces a space between the original value and the appended one. But
we can use this knowledge, to match any instance of a space followed by
a comma, and turn it into a single comma.

This allows users to now have a consistent use of the '=' and '+='
operators we use everywhere else in .mk files.

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