This is the uEnv.txt file I used to get yocto to boot.

uboot yocto uEnv.txt

##This will work with: Angstrom's 2013.06.20 u-boot.

loadaddr=0x82000000
fdtaddr=0x88000000
rdaddr=0x88080000

initrd_high=0xffffffff
fdt_high=0xffffffff

loadximage=load mmc 0:2 ${loadaddr} /boot/zImage-3.14.19-yocto-standard
loadxfdt=load mmc 0:2 ${fdtaddr} /boot/devicetree-zImage-am335x-boneblack.dtb
loadxrd=load mmc 0:2 ${rdaddr} /boot//initrd.img-${uname_r}; setenv rdsize ${filesize}
loaduEnvtxt=load mmc 0:1 ${loadaddr} uEnv.txt ; env import -t ${loadaddr} ${filesize};
loadall=run loaduEnvtxt; run loadximage; run loadxfdt;

mmcargs=setenv bootargs console=tty0 console=${console} ${optargs} ${cape_disable} ${cape_enable} root=${mmcroot} rootfstype=${mmcrootfstype} ${cmdli
ne}

uenvcmd=run loadall; run mmcargs; bootz ${loadaddr} - ${fdtaddr};