configs: am65x_evm: Switch envboot out for distro_bootcmd

Swap out the TI-centric "envboot" logic for the generic distro_bootcmd
logic for the bootcmd we run before trying to do something more complex
involving additional firmware, etc.

Cc: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Link: https://lore.kernel.org/r/20210713141139.15183-1-trini@konsulko.com