beaglebone: Find /boot partition on mmcblk0

After commit edcf39820f94c84b29c95a0d7b16b8d36857e87b the beaglebone
builds fail to mount the boot partition as it is specified in
/etc/fstab with the device node /dev/mmcblkp1.  With systemd in
particular this is considered an error and the system drops into
emergency mode.

(From meta-yocto rev: 11dbb98836768b71945817881709c5437d6eec6b)

Signed-off-by: Drew Moseley <drew.moseley@northern.tech>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta-yocto-bsp/wic/beaglebone.wks

@@ -2,5 +2,5 @@
 # long-description: Creates a partitioned SD card image for Beaglebone.
 # Boot files are located in the first vfat partition.
 
-part /boot --source bootimg-partition --ondisk mmcblk --fstype=vfat --label boot --active --align 4 --size 16
+part /boot --source bootimg-partition --ondisk mmcblk0 --fstype=vfat --label boot --active --align 4 --size 16
 part / --source rootfs --ondisk mmcblk --fstype=ext4 --label root --align 4