configs: am65x_evm: Use DEFAULT_LINUX_BOOT_ENV and remove overlayaddr

Use DEFAULT_LINUX_BOOT_ENV to define the standard addresses used in rest
of TI platforms as defined in ti_armv7_common.h

This avoids the standard pitfalls we've had with kernel images and fdt
addresses stomping on each other.

As part of this process, redefine overlayaddr to be dtboaddr (defined
in ti_armv7_common.h for this very purpose) and get rid of the
definition of overlayaddr..

Signed-off-by: Nishanth Menon <nm@ti.com>