configs: am62a: convert bootcmd to distro_bootcmd

We're currently using CONFIG_BOOTCOMMAND to run custom boot scripts to
jump into linux. While this works, let's begin the transition to more
distribution friendly jumps to linux by enabling distro_bootcmd.

Convert the custom bootcmd to a distro_bootcmd

Signed-off-by: Judith Mendez <jm@ti.com>
Signed-off-by: Bryan Brattlof <bb@ti.com>