sdimage-raspberrypi.wks: find /boot partition on mmcblk0

Recently wic was modified to no longer exclude /boot from partitions
added to fstab.  The --on parameter in many kickstart specifications
insufficiently resolved the MMC device, resulting in attempts to mount
/dev/mmcblkp1 as boot when the device should be /dev/mmcblk0p1.

With systemd the mount failure is an error and the system drops into
emergency mode.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>

wic/sdimage-raspberrypi.wks

@@ -2,5 +2,5 @@
 # long-description: Creates a partitioned SD card image for use with
 # Raspberry Pi. Boot files are located in the first vfat partition.
 
-part /boot --source bootimg-partition --ondisk mmcblk --fstype=vfat --label boot --active --align 4096 --size 20
-part / --source rootfs --ondisk mmcblk --fstype=ext4 --label root --align 4096
+part /boot --source bootimg-partition --ondisk mmcblk0 --fstype=vfat --label boot --active --align 4096 --size 20
+part / --source rootfs --ondisk mmcblk0 --fstype=ext4 --label root --align 4096