Hi, I wrote a CFG entry on the ucl2.xml file to mount a USB drive and dump the partitions I needed. I hope it helps you.
<LIST name="imx6-dump-mmcblk2" desc="Dump /dev/mmcblk2 into an ext4 USB drive">
<CMD state="BootStrap" type="boot" body="BootStrap" file ="u-boot_dl.bin" file1="u-boot_qd.bin" >Loading U-boot</CMD>
<CMD state="BootStrap" type="load" file="uImage" address="0x10800000"
loadSection="OTH" setSection="OTH" HasFlashHeader="FALSE" >Loading Kernel</CMD>
<CMD state="BootStrap" type="load" file="initramfs.cpio.gz.uboot" address="0x10C00000"
loadSection="OTH" setSection="OTH" HasFlashHeader="FALSE" >Loading Initramfs</CMD>
<CMD state="BootStrap" type="jump" >Jumping to OS image</CMD>
<!-- -->
<CMD state="Updater" type="push" body="$ ls -la /">List root</CMD>
<CMD state="Updater" type="push" body="$ ls -la /dev/sd*">List USB dev nodes</CMD>
<CMD state="Updater" type="push" body="$ mkdir -p /tmp/mntusb">Make mntusb directory</CMD>
<CMD state="Updater" type="push" body="$ mount -t ext4 /dev/sda1 /tmp/mntusb">Mount ext4 USB</CMD>
<CMD state="Updater" type="push" body="$ cat /dev/mmcblk2 > /tmp/mntusb/mmcblk2_dump">Dump eMMC</CMD>
<CMD state="Updater" type="push" body="$ ls -la /tmp/mntusb">List USB contents</CMD>
<CMD state="Updater" type="push" body="$ sync">Sync USB FS</CMD>
<!-- -->
<CMD state="Updater" type="push" body="$ echo Dump Complete!">Dump Complete</CMD>
</LIST>