linux-postmarketos-qcom-sm6125: new aport

This commit adds a package for the sm6125-mainline kernel fork. As of
now it uses kernel version 5.18_rc4, pulled from torvalds/linux.
You can find the repository on https://gitlab.com/sm6125-mainline/linux

linux-postmarketos-qcom-sm6125: Change commit to master branch

I'm planning to organize the sm6125-mainline project with the following
branch scheme:

- master: Stable release, commits here will be sent to LKML
- upstream: For pulling changes from torvalds/linux, testing and fixing
  prior to merging into master
- Any other branches will be used to implement new features to master

This commit switches the previously used commit in favor of the master branch.

linux-postmarketos-qcom-sm6125: Fixed checksums

linux-postmarketos-qcom-sm6125: Metadata adjustments

To be more exact, this commit:
- Changes the package version from 5.18.0 to 5.18_rc4, which is the
  current kernel version
- Specifies the location of the kernel config base