arm: mvebu: Remove "u-boot.kwb" CONFIG_BUILD_TARGET for non-SPL targets

By removing this default build target, the "u-boot.kwb" target is not
automatically generated. This fixes a temporary build error when using
out-of-tree builds, as buildman does per default (reported by Simon).

When the full SPL support is added for these targets with the DDR training
code, the "u-boot-spl.kwb" image will be generated automatically.

Users providing a special bin_hdr binary (binary.0) file can always add
this binary file and generate the full firmware image by issuing the
"make u-boot.kwb" command directly.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Cc: Prafulla Wadaskar <prafulla@marvell.com>
Cc: Luka Perkov <luka.perkov@sartura.hr>