recipetool: Use SPDX license identifiers

There are still a couple of cases where the license may be set as, e.g.,
"GPL" or "GPL-2.0" since there is not enough information to decide the
actual SPDX license. It is then assumed that the developer will have to
correct the information.

Signed-off-by: Peter Kjellerstedt <peter.kjellerstedt@axis.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>