found on eevBlog

To get a backup console access in case of fail – you need a RS232 accss to the unit

$>printenv bootargs
$>bootargs=root=PARTUUID=24d1deac-3434-1a4e-98d1-68ee2945a5f1 rootwait ro console=ttyS2,115200 rfkill.default_state=0
$>setenv bootargs 'root=PARTUUID=24d1deac-3434-1a4e-98d1-68ee2945a5f1 rootwait ro console=ttyS2,115200 rfkill.default_state=0 single init=/bin/sh'
$>boot
$>mount -rw -o remount /
$>passwd root
$>reboot -f

To list mmc :

=> mmc list
dwmmc@ff0f0000: 0 (eMMC)

to choose mmc dev :

=> mmc dev 0
switch to partitions #0, OK
mmc0(part 0) is current device

to get mmc info :

=> mmcinfo
Device: dwmmc@ff0f0000
Manufacturer ID: 45
OEM: 100
Name: DG400
Tran Speed: 52000000
Rd Block Len: 512
MMC version 4.0
High Capacity: Yes
Capacity: 7.3 GiB
Bus Width: 8-bit
Erase Group Size: 512 KiB