configs: j721e_evm: Use DEFAULT_LINUX_BOOT_ENV

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).. we will get rid of
overlayaddr later in the series.

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