Commit b6251db8 ("Kconfig: Introduce USE_BOOTCOMMAND and migrate
BOOTCOMMAND") removed CONFIG_BOOTCOMMAND option from
include/configs/am335x_evm.h file. But that option wasn't added to
defconfig files for BeagleBone Black board.

Because of this we can't boot Linux from SD card using just
"run bootcmd", getting next error:

    ** File not found /boot/undefined **

That's because "fdtfile" variable has "undefined" value by default, and
"bootcmd" doesn't call "run findfdt" command, which assigns "fdtfile" to
correct device tree file for current board name (obtained from EEPROM).

So we are forced to either call "run findfdt" command manually, or
assign manually "fdtfile=am335x-boneblack.dtb" (e.g. in uEnv.txt file).

Bring back CONFIG_BOOTCOMMAND to BBB defconfigs so that we can boot
Linux rootfs from SD card automatically without any addition actions.
Signed-off-by: NSam Protsenko <semen.protsenko@linaro.org>