uboot:scripts: Change slot names to match bootselect names

Historically, we use the names "alternative", "regular" and "regularB" to name the three operating systems on the boards. The bootselect tool also sets these names in the U-Boot ENV. Change the boot script to align with these names, too.

requested review from @oleksii.kutuzov

assigned to @tobias.poganiuch

⮘ seconorth-wayland | seco-mx6 ⮚
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⮘ seconorth-wayland | seco-mx6ull ⮚
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⮘ seconorth-wayland | seco-mx8mm ⮚
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⮘ seconorth-wayland | seco-mx8mp ⮚
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⮘ seconorth-wayland | seco-genio700 ⮚
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⮘ seconorth-wayland | seco-genio510 ⮚
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⮘ seconorth-fngsystem | seco-mx6 ⮚
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⮘ seconorth-fngsystem | seco-mx6ull ⮚
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⮘ seconorth-fngsystem | seco-mx8mm ⮚
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⮘ seconorth-fngsystem | seco-mx8mp ⮚
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⮘ seconorth-fngsystem | seco-genio700 ⮚
├── artifacts.zip size: [ 203.01 MiB → 203.01 MiB ] | +161 B
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⮘ seconorth-fngsystem | seco-genio510 ⮚
├── artifacts.zip size: [ 203.09 MiB → 203.09 MiB ] | +79 B
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approved this merge request

I will adjust u-boot as well and merge both at the same time

Corresponding change for u-boot: seco-ne/3rd-party/u-boot/u-boot-mtk!34 (merged)

merged

Integration merge request: seco-ne/yocto/manifest!1826 (merged)

mentioned in merge request seco-ne/yocto/manifest!1826 (merged)