Replace 'x-hack' with direct comparison operators

[sweep-nightly]

Description

This PR replaces the 'x-hack' comparison method with direct comparison operators in the following files:

• lib/functions/artifacts/artifact-armbian-base-files.sh
• lib/functions/artifacts/artifacts-reversion.sh
• lib/functions/artifacts/artifact-uboot.sh
• lib/functions/artifacts/artifacts-obtain.sh
• lib/functions/rootfs/distro-specific.sh

The 'x-hack' method is used in shell scripting to prevent errors when comparing variables that could be potentially undefined or empty. However, in modern bash scripting, direct comparison operators can handle these cases and are more readable and straightforward.

Summary of Changes

• Replaced all instances of 'x-hack' with direct comparison operators in the mentioned files.
• Ensured that the replacement does not affect the logic of the code.

This PR contributes to the resolution of issue #83.

Fixes #83.

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