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/cpu/Makefile b/arch/riscv/cpu/Makefile
index 63de163..2cc6757 100644
--- a/arch/riscv/cpu/Makefile
+++ b/arch/riscv/cpu/Makefile
@@ -2,4 +2,6 @@
 #
 # Copyright (C) 2018, Bin Meng <bmeng.cn@gmail.com>
 
+extra-y = start.o
+
 obj-y += cpu.o