arm: unify linker script

all arm boards except a few use the same cpu linker script
so move it to cpu/$(CPU)

that could be overwrite in following order
SOC
BOARD
via the corresponding config.mk

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
diff --git a/board/samsung/smdk6400/config.mk b/board/samsung/smdk6400/config.mk
index 0255a07..4ab1d7e 100644
--- a/board/samsung/smdk6400/config.mk
+++ b/board/samsung/smdk6400/config.mk
@@ -28,3 +28,5 @@
 else
 TEXT_BASE = 0
 endif
+
+LDSCRIPT := $(SRCTREE)/board/$(BOARDDIR)/u-boot-nand.lds