create-spdx.bbclass: Search all license directories for licenses

Before, even if the code was seemingly written to search through all
licenses in ${COMMON_LICENSE_DIR} and ${LICENSE_PATH}, it would
actually bail out after only searching ${COMMON_LICENSE_DIR} due to
the exception handling.

Also refrain from using f-strings.

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