spdx30: Provide software_packageUrl field in SPDX 3.0 SBOM

spdx30: Provide software_packageUrl field in SPDX 3.0 SBOM

Define var-SPDX_PACKAGE_URL to provide software_packageUrl field [1][2]
in SPDX 3.0 SBOM, support to override with package name
SPDX_PACKAGE_URL:<pkgname>

Currently, the format of purl is not defined in Yocto, set empty for now
until we have a comprehensive plan for what Yocto purls look like.
But users could customize their own purl by setting var-SPDX_PACKAGE_URL

[1] https://spdx.github.io/spdx-spec/v3.0.1/model/Software/Properties/packageUrl/
[2] https://spdx.github.io/spdx-spec/v3.0.1/annexes/pkg-url-specification/

(From OE-Core rev: c8e6953a0b6f59ffca994c440069db39e60b12d2)

Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Peter Marko <peter.marko@siemens.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>