nios2: Make STANDALONE_LOAD_ADDR configurable per board

Follow commit 8ae86b76
which changed the variable name.

Fix this error,
nios2-elf-ld: invalid hex number `-o'
Signed-off-by: NThomas Chou <thomas@wytron.com.tw>
Signed-off-by: NScott McNutt <smcnutt@psyent.com>

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