bootvx: x86: Prepare e820 related stuff from the given kernel memory base address

At present two environment variables 'e820data'/'e820info' are required
to boot a VxWorks x86 kernel, but this is superfluous. The offset of
these two tables are actually at a fixed offset from the kernel memory
base address and we can provide the kernel memory base address to U-Boot
via only one variable 'vx_phys_mem_base'.

Note as it name indicates, the physical address should be provided.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>