ti: omap5: Switch to generic distro boot for non-Android cases

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.  Note that if we do not find anything
here we will continue to try and Android methods and will start by
trying fastboot.

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/20210701132612.7559-3-trini@konsulko.com