configs: j7200_evm_a72_defconfig: Switch to bootstd

Switch to using bootstd. Note with this change, we will stop using
distro_bootcmd and instead depend entirely on bootflow method of
starting the system up.

Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Neha Malcom Francis <n-francis@ti.com>