wic: sdimage-bootpart: Use mmcblk0 drive instead of bogus mmcblk
Apparently either nobody ever used sdimage-bootpart.wks
or fstab was kept untouched due to "--no-fstab-update" usage as
some boards like IMX may have, see [1].
Otherwise addition of the following line to the target's fstab:
---------------------->8-----------------
/dev/mmcblkp1 /boot vfat defaults 0 0
---------------------->8-----------------
gets us unusable target:
---------------------->8-----------------
[* ] A start job is running for dev-mmcblkp1.device (23s / 1min 30s)
[ TIME ] Timed out waiting for device dev-mmcblkp1.device.
[DEPEND] Dependency failed for /boot.
[DEPEND] Dependency failed for Local File Systems.
You are in emergency mode. After logging in, type "journalctl -xb" to view
system logs, "systemctl reboot" to reboot, "systemctl default" or "exit"
to boot into default mode.
Press Enter for maintenance
---------------------->8-----------------
