Filter real devices in `findmnt`.

[SpacingBat3]

This PR ticket replaces #5, as #5 used master branch on my forked repository. However due I might commit more changes in the future, I had to create a separate branch for this PR so it won't contain changes not being scope of this PR.

Original description of PR from #5:

Currently, kesboot will pick any file system available first at the list, including virtual ones, for BOOT_DIR and ROOT_DEVICE. This causes on some workflows to pick file system like systemd-1 instead of path to real block device.

This PR aims to fix that by using --real flag within findmnt.