gpt-auto-generator: skip nonexistent devices

gpt-auto-generator: skip nonexistent devices

The devices we work with have eMMC chips for storage. The chips
provide four "hardware" partitions.  The first is /dev/mmcblk0, it
takes almost whole space and holds a GPT with several real partitions
(/dev/mmcblk0p?). Then there are three block devices (mmcblk0boot0,
mmcblk0boot1, rpmb) that are part of the same hardware as mmcblk0 that
are presented by the kernel as children of the latter. That relationship
makes gpt-auto-generator try to peek them but since they are not GPT
partitions blkid_do_safeprobe() returns -2 making verify_gpt_parition()
function return -ENODEV.