cmd: bootvx: Pass E820 information to an x86 VxWorks kernel

E820 is critical to the kernel as it provides system memory map
information. Pass it to an x86 VxWorks kernel.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
Tested-by: Jian Luo <jian.luo4@boschrexroth.de>