nios2: Make STANDALONE_LOAD_ADDR configurable per board

Follow commit 8ae86b76c648d8bef97241c78e2fddb1c4f164d3
which changed the variable name.

Fix this error,
nios2-elf-ld: invalid hex number `-o'

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 e58ea24..7b03ed8 100644
--- a/arch/nios2/config.mk
+++ b/arch/nios2/config.mk
@@ -24,7 +24,7 @@
 
 CROSS_COMPILE ?= nios2-elf-
 
-STANDALONE_LOAD_ADDR ?= 0x02000000
+CONFIG_STANDALONE_LOAD_ADDR ?= 0x02000000
 
 PLATFORM_CPPFLAGS += -DCONFIG_NIOS2 -D__NIOS2__
 PLATFORM_CPPFLAGS += -G0