U-Boot# printenv snappy_ab
snappy_ab=b
U-Boot# reset
resetting ...

U-Boot SPL 2015.07-dirty (Jul 17 2015 - 11:40:49)
MMC: block number 0x100 exceeds max(0x0)
MMC: block number 0x200 exceeds max(0x0)
*** Error - No Valid Environment Area found
*** Warning - bad CRC, using default environment



U-Boot 2015.07-dirty (Jul 17 2015 - 11:40:49 +0200)

       Watchdog enabled
I2C:   ready
DRAM:  512 MiB
MMC:   OMAP SD/MMC: 0, OMAP SD/MMC: 1
reading uboot.env
Net:   <ethaddr> not set. Validating first E-fuse MAC
cpsw, usb_ether
Hit any key to stop autoboot:  0 
switch to partitions #0, OK
mmc0 is current device
SD/MMC found on device 0
reading boot.scr
** Unable to read file boot.scr **
reading uEnv.txt
0 bytes read in 4 ms (0 Bytes/s)
Loaded environment from uEnv.txt
Importing environment from mmc ...
Running uenvcmd ...
reading a/vmlinuz
6517648 bytes read in 386 ms (16.1 MiB/s)
reading a/initrd.img
13546197 bytes read in 797 ms (16.2 MiB/s)
reading a/dtbs/am335x-boneblack.dtb
30025 bytes read in 13 ms (2.2 MiB/s)
Kernel image @ 0x82000000 [ 0x000000 - 0x637390 ]
## Flattened Device Tree blob at 88000000
   Booting using the fdt blob at 0x88000000
   Loading Ramdisk to 8f314000, end 8ffff2d5 ... OK
   Loading Device Tree to 8f309000, end 8f313548 ... OK