Remove 'x-hack' from grub and bsp packages

[sweep-nightly]

Description

This PR removes the usage of 'x-hack' in the grub and bsp packages of the armbian-build repository. The 'x-hack' is a common practice in shell scripting for string comparison to avoid syntax errors when one of the variables is empty or unset. However, it is not necessary when using the double bracket conditional construct [[ ]], which can handle empty variables. The 'x-hack' comparisons have been replaced with direct comparisons using double brackets [[ ]].

Summary of Changes

• In packages/bsp/nanopim4/nanopim4-pwm-fan.sh, replaced all instances of 'x-hack' comparisons with direct comparisons using double brackets [[ ]].
• In packages/blobs/grub/09_linux_with_dtb.sh, replaced all instances of 'x-hack' comparisons with direct comparisons using double brackets [[ ]].

Please review the changes and merge this PR if it meets the requirements. Thank you!

Fixes #19.

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