[U-Boot] Targets for BeagleBones

I assume that the most appropriate configuration target for BeagleBone Black is am335x_boneblack_config. But what about BeagleBone White? Up until now we've been using am335x_evm_config, but are considering switching to the former.

This discussion started after the commit 80b24fcd3 in U-Boot broke some MMC compatibility on the BeagleBone Black (specifically, accessing the internal MMC when holding the button to boot from external MMC). This was due to the addition of the CONFIG_DM_MMC option. Since this still works with the am335_boneblack_config target, we're considering the switch for both boards.