device-{kobo,tolino}-*: workaround GPT handling bug in pmbootstrap

Bootloader needs to be at 0x400 for IMX6 devices. That is ignored by pmbootstrap which places the GPT header there which then get overwritten when writing the bootloader image. As soon as partition table is touched (e.g. to enlarge the root partition), that GPT header is restored by the backup GPT header. Result: the device only boots once. Revert to msdos partition table style to workaround that bug.

Reason for being draft: partition table is still created as GPT despite following the note in https://wiki.postmarketos.org/wiki/Porting_to_a_new_device#Installation