cmd: mvebu/bubt: move eMMC data-partition uboot from LBA-0 to 4096

A38x bootrom only searches 2 sectors when booting from eMMC,
irregardless of data or boot partition: 0 & 4096.

For eMMC boot partitions sector 0 is fine, but on data partition it
conflicts with MBR.

Change bubt command default to 4096 for eMMC data partition only, to
allow using an MBR partition table on the eMMC data partition while also
booting from it.

Signed-off-by: Josua Mayer <josua@solid-run.com>
Reviewed-by: Stefan Roese <sr@denx.de>