meta, meta-selftest: Replace more non-SPDX license identifiers

In commit ceda3238 (meta/meta-selftest/meta-skeleton: Update LICENSE
variable to use SPDX license identifiers) all LICENSE variables were
updated to only use SPDX license identifiers.

This does the same for comments and other variables where it is
appropriate to use the official SPDX license identifiers. There are
still references to, e.g., "GPLv3", but they are then typically in
descriptive text where they refer to the license in a generic sense.

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