ti: am43xx_evm: Switch to DISTRO_BOOT only

Remove the environment support for various legacy boot methods.  With
this, we will now default to booting any distribution that follows the
generic distro boot framework and no longer attempt to boot various
legacy (to this SoC) scripts/etc.

Signed-off-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Link: https://lore.kernel.org/r/20210701132612.7559-1-trini@konsulko.com