bootvx: x86: Make VxWorks EFI console driver happy

When booting from EFI BIOS, VxWorks bootloader stores the EFI GOP
framebuffer info at a pre-defined offset @ 0x6100. When VxWorks
kernel boots up, its EFI console driver tries to find such a block
and if the signature matches, the framebuffer information will be
used to initialize the driver.

However it is not necessary to prepare an EFI environment for
VxWorks's EFI console driver to function (eg: EFI loader in
U-Boot). If U-Boot has already initialized the graphics card and
set it to a VESA mode that is compatible with EFI GOP, we can
simply prepare such a block for VxWorks.

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