mvebu: select boot device at SoC level

Move the gdsys Controlcenter DC specific build time kwbimage.cfg
generation code into the mach-mvebu/ directory to be shared by all 32bit
mvebu platforms.

Remove board specific kwbimage.cfg files, and use the generated one
instead. These files are all identical, with two exceptions. Clearfog
and Helios4 use the sdio boot device, whereas all others use spi. Update
the defconfigs for the exceptional boards to generate the same
kwbimage.cfg as before.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Stefan Roese <sr@denx.de>