arm: mvebu: Generate kwbimage.cfg with $(call cmd, ...)

arm: mvebu: Generate kwbimage.cfg with $(call cmd, ...)

Usage of $(call cmd,...) is standard way to call other commands which
generate things.

It also has the advantage of printing build information in the form
  KWBCFG  arch/arm/mach-mvebu/kwbimage.cfg
if verbosity is disabled, and printing the build command otherwise.

Note that the '#' character needs to be escaped in Makefile when used as
value for make variable assignment.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>