postmarketos-update-kernel: support A/B devices

The "magic" line here:

grep -o 'androidboot\.slot_suffix=..' /proc/cmdline |  cut -d "=" -f2

Checks for the kernel cmdline argument androidboot.slot_suffix and gets its value. This cmdline arg is required for all A/B devices [1]. On non A/B devices it will be empty.

This will enable seamless kernel updates for the OnePlus 6, 6T and a lot of other modern devices (MSM8998+) devices with slots which are slowly starting to appear in pmaports.


Edited by Caleb Connolly

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