arm/km: fix u-boot.kwb build breakage

arm/km: fix u-boot.kwb build breakage

commit 010a958b
(arm/km: remove CONFIG_SYS_KWD_CONFIG from keymile-common.h)
breaks building keymile arm targets, when u-boot.kwb tries to
generate the binary with mkimage. A simple make <board> or MAKEALL
succeeded because it don't try to build the kirwood binary at the end.

Due this commit we use the CONFIG_SYS_KWD_CONFIG from the
arch-kirkwood/config.h and it was removed from the board config.
But it was forgotten to include the header. Now the header is included
in km_arm.h. Some other defines were obsolete due to this include,
these are also removed in this commit.

Signed-off-by: Holger Brunck <holger.brunck@keymile.com>
cc: Valentin Longchamp <valentin.longchamp@keymile.com>
cc: Prafulla Wadaskar <prafulla@marvell.com>
cc: Heiko Schocher <hs@denx.de>