boot_device method in features/step_definitions/usb.rb is broken

It happens to see this kind of failures in Jenkins:

features/usb_install.feature:2
  Error trace:
    We are running from device /dev/sda, but for usb drive 'isohybrid' we expected to run from either device /dev/sda1 (when installed via the USB installer) or /dev/sda1 (when installed from an isohybrid).

It seems to happen when isohybrids are used. If so config/chroot_local_includes/lib/live/config/998-permissions may also be broken. It reappeared on some new builds.

Feature Branch: test/10504-allow-more-boot_devices

Parent Task: #10288

Related issues

Related to #11083 (closed)

Original created by @bertagaz on 10504 (Redmine)

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