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/actux3/config.mk b/board/actux3/config.mk
index 9a634cd..119140d 100644
--- a/board/actux3/config.mk
+++ b/board/actux3/config.mk
@@ -2,3 +2,5 @@
 
 # include NPE ethernet driver
 BOARDLIBS = cpu/ixp/npe/libnpe.a
+
+LDSCRIPT := $(SRCTREE)/board/$(BOARDDIR)/u-boot.lds