all: add HiFive1 rev B board with RISC-V architecture

This page has been a big help in adding support for this new chip:
https://wiki.osdev.org/HiFive-1_Bare_Bones

targets/fe310.json

@@ -0,0 +1,5 @@
+{
+	"inherits": ["riscv"],
+	"features": ["+a", "+c", "+m"],
+	"build-tags": ["fe310", "sifive"]
+}