Changes to enable SMP kernels in riscv-poky with Spike simulation

[jleidel]

These changes enable SMP builds by default for the RISC-V kernel. It also changes the runspike script to permit users to pass arguments to Spike using the SPIKE_ARGS environment variable. This would permit users to quickly and easily boot and execute functional RISC-V SMP development environments. The README.md documentation has been updated to reflect the change.