riscv: Make start.S available for all targets

Currently start.S is inside arch/riscv/cpu/ax25/, but it can be
common for all RISC-V targets.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
index af432e1..8fb6a88 100644
--- a/arch/riscv/Makefile
+++ b/arch/riscv/Makefile
@@ -3,7 +3,7 @@
 # Copyright (C) 2017 Andes Technology Corporation.
 # Rick Chen, Andes Technology Corporation <rick@andestech.com>
 
-head-y := arch/riscv/cpu/$(CPU)/start.o
+head-y := arch/riscv/cpu/start.o
 
 libs-y += arch/riscv/cpu/
 libs-y += arch/riscv/cpu/$(CPU)/