styhead migration fixes
Based on the guidance from [1], resolve build errors on styhead.
This resolves 2 issues with styhead as found with the integration at [2]
and testing of branch at [3], as referenced at [4], [5], [6], and [7]:
```
WARNING: swupdate-image-tegra-1.0-r0 do_unpack: swupdate-image-tegra: the directory ${WORKDIR}/${PN} (/home/dan/trellis/oe4t/tegra-demo-distro/build-master/tmp/work/jetson_orin_nano_devkit_nvme-oe4t-linux/swupdate-image-tegra/1.0/swupdate-image-tegra) pointed to by the S variable doesn't exist - please set S within the recipe to point to where the source has been unpacked to
```
This warning is printed due to the setting of S based on WORKDIR
in swupdate.bbclass and the fact that the S directory is no longer
automatically created as discussed in [1]. To resolve, use the guidance
in [1] to set S and UNPACKDIR for recipes which only use file:// sources.
In addition, as suggested in [7], move to swupdate-common to share between
swupdate-image and swupdate.
```
Exception: FileNotFoundError: [Errno 2] No such file or directory: '/home/dan/trellis/oe4t/tegra-demo-distro/build-master/tmp/work/jetson_orin_nano_devkit_nvme-oe4t-linux/swupdate-image-tegra/1.0/swupdate-image-tegra/sw-description'
```
This error occurs due to a missing S directory which is no longer created
automatically after styhead
When using the guidance in [1] to set UNPACKDIR there is no
longer a need to copy from WORKDIR to UNPACKDIR since S is UNPACKDIR, so both
the workdir reference and shutil copy can be removed.
1: https://docs.yoctoproject.org/next/migration-guides/migration-5.1.html
2: https://github.com/OE4T/tegra-demo-distro/blob/master/layers/meta-tegrademo/dynamic-layers/meta-swupdate/README.md
3: https://github.com/Trellis-Logic/tegra-demo-distro/tree/master%2Bswupdate-kas
4: https://groups.google.com/g/swupdate/c/E2h_qgHvJuk
5: https://groups.google.com/g/swupdate/c/x4jxRPbtXa0/m/QQkYqgILAQAJ
6: https://groups.google.com/g/swupdate/c/x4jxRPbtXa0
7: https://groups.google.com/g/swupdate/c/8K-9H7C9o5E
Signed-off-by: Dan Walkes <danwalkes@trellis-logic.com>
Dan Walkes