configs: am57xx_evm_defconfig: Enable 'bcb' command

It is essential to have an access to BCB area of 'misc' partition on
Android devices [1]. For BeagleBoard X15 the 'bcb' command will be
further used for reboot reason implementation and booting to recovery.
It can be also used for debugging reasons, like checking RescueParty
messages in BCB area.

[1] doc/android/bcb.txt

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>