Add kernel command line option for spike

It is very inconvenient to always embed kernel flat image into
OpenSBI for booting Linux/Xvisor on Spike.

We add optional "--kernel" command line option for spike. Using
this new option, users can specify kernel flat image separately
and OpenSBI ELF separately.

Signed-off-by: Anup Patel <anup.patel@wdc.com>