x86: doc: Update to include Intel Bayley Bay board instructions

Update existing documentation to mention Intel Bayley Bay board
instructions, an additional Bay Trail based board to MinnowMax.

This also adds a minor change to QEMU section to indicate clearly
the instructions are for bare mode.

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