arm: mvebu: Avoid generating kwbimage.cfg in the source tree

At present some boards generate kwbimage.cfg in the source tree
during the build. This breaks buildman testing on some systems
where the source tree is read-only. Update makefile rules to
generate it in the build tree instead.

Note some other boards have the kwbimage.cfg file written in
advance, hence we need check if the file exists in the build
tree first, otherwise we fall back to one in the source tree.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Chris Packham <judge.packham@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>

arm: mvebu: Add DB-XC3-24G4XG board

The DB-XC3-24G4XG is a switch development board from Marvell. It can
either use and external CPU card such as the db-88f6820-amc or the
internal CPU that is integrated into the switch.

Add support for running U-Boot on the internal CPU and enable the USB,
SPI and NAND peripherals. For now this needs the bin_hdr from the
Marvell U-Boot for this board.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Stefan Roese <sr@denx.de>

arm: mvebu: Add DB-XC3-24G4XG board

The DB-XC3-24G4XG is a switch development board from Marvell. It can
either use and external CPU card such as the db-88f6820-amc or the
internal CPU that is integrated into the switch.

Add support for running U-Boot on the internal CPU and enable the USB,
SPI and NAND peripherals. For now this needs the bin_hdr from the
Marvell U-Boot for this board.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Stefan Roese <sr@denx.de>