beagle: beagley-ai: env: Set up the boot devices and boot methods

beagle: beagley-ai: env: Set up the boot devices and boot methods

We just have a single MMC on BeagleY-AI. So drop all other boot options,

In the meanwhile, we also will use efiboot and standard boot methods
consistent with other K3 Beagle products.

Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Udit Kumar <u-kumar1@ti.com>
Reviewed-by: Bryan Brattlof <bb@ti.com>
Reviewed-by: Andrew Davis <afd@ti.com>