uc20: add dtbo partition support for lk boot loader

[kubiko]

Some devices use dtbo image, which is stored in separate partition from where boot image is This requires lk bootloader plugin to extract correct file from the kernel snap to the correct partition.

We use spare slots in lkenv to define mapping matrix(Bootimg_to_dtboimg_matrix). This tells snapd which dtbo_* partition is to be used for which boot_* partition. Additionally there is also value for dtbo filename (Dtboimg_file_name). Since this is optional feature, defining value for Dtboimg_file_name is used to control if in use.

Both Dtboimg_file_name and Bootimg_to_dtboimg_matrix are static and defined at gadget build time. Snapd does not change or alter any of those values.

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[Meulengracht]

@kubiko is this still relevant?

[ernestl]

Discussed with @kubiko , this is still relevant and should not be removed.