x86: fsp: Make hob command a sub-command to fsp

Introduce a new fsp command and make the existing hob command a
sub-command to fsp for future extension. Also move cmd_hob.c to
the dedicated fsp sub-directory in arch/x86/lib.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>