xes: Make X-ES board names more generic

Some U-Boot images for X-ES boards support multiple products in the same
family.  For example, the XPedite5370, XPedite5371, and XPedite5372 are
similar enough that one U-Boot image can work on all 3 cards.  To make it
clear that a U-Boot image can work on boards of the same family, rename
the boards with the least significant digit of 'x'.

While we're at it, change the board config file and make targets to be
lowercase.

Also change the default uImage and fdt filenames to "board.uImage" and
"board.dtb" to be more generic.

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>