Build for HiFive1

Debugging:

On first console, run

JLinkGDBServer -singlerun -if JTAG -select USB -speed 1000 -jtagconf -1,-1 -device FE310 --port 2331

On second console, run riscv64-unknown-elf-gdb:

file ".pio/build/hifive1-revb/firmware.elf"
target extended-remote :2331
b main
monitor halt
monitor reset
load
c