nios2: allow link script overriding from boards

This patch allow boards to override the default link script.

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Signed-off-by: Scott McNutt <smcnutt@psyent.com>

arch/nios2/config.mk

diff --git a/arch/nios2/config.mk b/arch/nios2/config.mk
index f455982..8e5d6ef 100644
--- a/arch/nios2/config.mk
+++ b/arch/nios2/config.mk
@@ -29,4 +29,4 @@
 PLATFORM_CPPFLAGS += -DCONFIG_NIOS2 -D__NIOS2__
 PLATFORM_CPPFLAGS += -ffixed-r15 -G0
 
-LDSCRIPT := $(SRCTREE)/$(CPUDIR)/u-boot.lds
+LDSCRIPT ?= $(SRCTREE)/$(CPUDIR)/u-boot.lds