riscv: qemu: define standalone load address

We need to define the standalone load address to use standalone
application on qemu-riscv. Define it and set it equal to
CONFIG_SYS_LOAD_ADDR.

To not overwrite it, change the assigned of CONFIG_STANDALONE_LOAD_ADDR
in arch/riscv/config.mk to a conditional one.

Signed-off-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/arch/riscv/config.mk b/arch/riscv/config.mk
index e484a3f..84654eb 100644
--- a/arch/riscv/config.mk
+++ b/arch/riscv/config.mk
@@ -23,7 +23,7 @@
 EFI_LDS			:= elf_riscv64_efi.lds
 endif
 
-CONFIG_STANDALONE_LOAD_ADDR = 0x00000000
+CONFIG_STANDALONE_LOAD_ADDR ?= 0x00000000
 
 PLATFORM_CPPFLAGS	+= -ffixed-gp -fpic
 PLATFORM_RELFLAGS	+= -fno-common -gdwarf-2 -ffunction-sections \