include: am654_evm: Establish initial environment for SD card boot

include: am654_evm: Establish initial environment for SD card boot

Create a basic U-Boot environment that allows the automatic loading
of a Linux Kernel located at /boot/Image and an associated device tree
blob located at /boot/k3-am654-base-board.dtb from the secondary
partition of an ext4-formatted SD card on the AM654x EVM. Furthermore
the boot.scr and uEnv.txt detection and loading schemes are supported
in a manner already known from other TI platforms.

Note that this is intended to be a starting point to enable initial
board use and will most certainly get extended and refactored as
different boot media become available.

Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Andreas Dannenberg <dannenberg@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>