x86: Move commands from under arch/x86 to cmd/x86/

We only need to compile and link these files when building for full
U-Boot.  Move them to under cmd/x86/ to make sure they aren't linked in
and undiscarded due to u_boot_list_2_cmd_* being included).

Cc: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>