(BeagleBoneBlack)ubuntu@localhost:~$ cat  /boot/uboot/uEnv.txt 
# where to load initrd
initrd_addr=0x88080000
# load Snappy environment and call into Snappy boot after processing this file
uenvcmd=load mmc ${bootpart} ${loadaddr} snappy-system.txt; env import -t $loadaddr $filesize; run snappy_boot
# boot from SD, ignoring eMMC (LP: #1464275)
bootpart=0:1
(BeagleBoneBlack)ubuntu@localhost:~$ cat  /boot/uboot/snappy-system.txt 
# This is a snappy variables and boot logic file and is entirely generated and
# managed by Snappy. Modifications may break boot
######
# functions to load kernel, initrd and fdt from various env values
loadfiles=run loadkernel; run loadinitrd; run loadfdt
loadkernel=load mmc ${mmcdev}:${mmcpart} ${loadaddr} ${snappy_ab}/${kernel_file}
loadinitrd=load mmc ${mmcdev}:${mmcpart} ${initrd_addr} ${snappy_ab}/${initrd_file}; setenv initrd_size ${filesize}
loadfdt=load mmc ${mmcdev}:${mmcpart} ${fdtaddr} ${snappy_ab}/dtbs/${fdtfile}

# standard kernel and initrd file names; NB: fdtfile is set early from bootcmd
kernel_file=vmlinuz
initrd_file=initrd.img
fdtfile=am335x-boneblack.dtb

# extra kernel cmdline args, set via mmcroot
snappy_cmdline=init=/lib/systemd/systemd ro panic=-1 fixrtc

# boot logic
# either "a" or "b"; target partition we want to boot
snappy_ab=b
# stamp file indicating a new version is being tried; removed by s-i after boot
snappy_stamp=snappy-stamp.txt
# either "regular" (normal boot) or "try" when trying a new version
snappy_mode=regular
# if we're trying a new version, check if stamp file is already there to revert
# to other version
snappy_boot=if test "${snappy_mode}" = "try"; then if test -e mmc ${bootpart} ${snappy_stamp}; then if test "${snappy_ab}" = "a"; then setenv snappy_ab "b"; else setenv snappy_ab "a"; fi; else fatwrite mmc ${mmcdev}:${mmcpart} 0x0 ${snappy_stamp} 0; fi; fi; run loadfiles; setenv mmcroot /dev/disk/by-label/system-${snappy_ab} ${snappy_cmdline}; run mmcargs; bootz ${loadaddr} ${initrd_addr}:${initrd_size} ${fdtaddr}