x86: Update README to explain booting Ubuntu on Minnowmax

The steps required to boot a Linux distribution from U-Boot on x86 are not
very complicated, but it is a good idea to have these written down in an
accessible place.

Document how to examine the boot media from U-Boot, how to load a kernel,
load a ramdisk, set the kernel boot arguments and start the kernel. With
these instructions Ubuntu boots mostly normally on Minnowmax.

Note that the TSC timer does not operate correctly and gives warnings in
the boot log. I expect that ACPI support will solve this.

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