kirkwood: get rid of config.mk files

After moving the definition of CONFIG_SYS_TEXT_BASE to the respective
board config files, all Marvell kirkwood board have just a single and
common entry in their config.mk files:

	KWD_CONFIG = $(SRCTREE)/board/$(BOARDDIR)/kwbimage.cfg

Replace the only reference to KWD_CONFIG in the top level Makefile by
an equivalent setting, and remove all kirkwood config.mk files.

Signed-off-by: Wolfgang Denk <wd at denx.de>
Cc: Prafulla Wadaskar <prafulla at marvell.com>
Cc: Siddarth Gore <gores at marvell.com>
Cc: Simon Kagstrom <simon.kagstrom at netinsight.net>
Cc: Heiko Schocher <hs at denx.de>
Cc: Eric Cooper <ecc at cmu.edu>
Acked-by: Wolfgang Denk <wd at denx.de>
Signed-off-by: Prafulla Wadaskar <prafulla@marvell.com>