Use EROFS for the root filesystem (#3456)

* Use name.sh functions for paths in genimage

Paths for images generated outside of genimage were not used in genimage
definitions. Use them as the single source of truth.

Images generated by genimage itself (e.g. kernel.img) don't need to use those
functions, so remove the unused ones.

* Use EROFS instead of SquashFS for root FS

* Enabled EROFS in common kernel fragment
* RootFS image switched to EROFS with options to get decent compression
* rootfstype removed from kernel command line

* Get size of correct FS image in GH build summary

.github/workflows/build.yaml

@@ -316,7 +316,7 @@ jobs:
           echo "## Partitions" >> $GITHUB_STEP_SUMMARY
           echo "| File | Size (bytes) | Size (formatted) |" >> $GITHUB_STEP_SUMMARY
           echo "|:-|:-|:-|" >> $GITHUB_STEP_SUMMARY
-          for f in boot.vfat kernel.img rootfs.squashfs overlay.ext4 data.ext4; do
+          for f in boot.vfat kernel.img rootfs.erofs overlay.ext4 data.ext4; do
             echo "| ${f} | $(du -b output/images/$f | cut -f1) | $(du -bh output/images/$f | cut -f1) |" >> $GITHUB_STEP_SUMMARY
           done