bootvx: x86: Explicitly clear the bootloader image size

VxWorks bootloader stores its size at a pre-defined offset @ 0x5004.
Later when VxWorks kernel boots up and system memory information is
retrieved from the E820 table, the bootloader size will be subtracted
from the total system memory size to calculate the size of available
memory for the OS.

Explicitly clear the bootloader image size otherwise if memory
at this offset happens to contain some garbage data, the final
available memory size for the kernel is insane.

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