scripts: Add a conversion script to use SPDX license names

We're standardising on SPDX license names so it is overdue to change
the ones in the LICENSE fileds to the SPDX values. Add a conversion
script which makes this straightforward on the most part.

Ultimately this allows the core code to be more efficient and not need
to consult the mappings.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>