uboot-tools: add option for dumpimage

The tool dumpimage extracts data from U-Boot images.
U-Boot added dumpimage tool on release 2014.01.
This commit allows installation on target and host.

Signed-off-by: Paulo Zaneti <paulo.zaneti@datacom.ind.br>
Tested-by: Carlos Santos <casantos@datacom.ind.br>
Signed-off-by: Carlos Santos <casantos@datacom.ind.br>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>