When BOARD=fomu, use the riscv cross-compiler. Otherwise, use the
default arm compiler. This can be overridden by passing
CROSS_COMIPLE on the command line.
Note that there are now three common risc-v prefixes:
- riscv32-unknown-elf- : Common for users who compile their own
- riscv64-unknown-elf- : Upstream multiarch toolchain from SiFive
- riscv-none-embed- : xPack embedded version of SiFive toolchain
Here we assume users are using the `riscv-none-embed-` toolchain from
xPack, because it appears to be growing more common. Additionally,
there is much confusion surrounding `riscv64-unknown-elf-`, which
actually includes both 32- and 64-bit runtimes and can generate software
for both.
Signed-off-by: Sean Cross <sean@xobs.io>
