include: configs: am64x_evm: Add env variables for booting to kernel using USB MSC device

Add env variables for booting to kernel from USB MSC device. The second
partition in the USB MSC device needs to formatted as ext4 file system with
kernel and dtb images, present in the /boot folder.

Signed-off-by: Aswath Govindraju <a-govindraju@ti.com>