km/common: use u-boot.kwb for u-boot update function on arm

Now we use the standard u-boot make to build the Kirkwood binary.
The output file is u-boot.kwb. So use this name for the tftp
update function to avoid confusion, because this is the binary we
need on Kirkwood.

Signed-off-by: Holger Brunck <holger.brunck@keymile.com>
cc: Valentin Longchamp <valentin.longchamp@keymile.com>
cc: Heiko Schocher <hs@denx.de>
cc: Wolfgang Denk <wd@denx.de>