x86: Add back cold- and warm-boot flags

These were removed, but actually are useful.

Cold means that we started from a reset/power on.
Warm means that we started from another U-Boot.

We determine whether u-boot on x86 was warm or cold booted (really if
it started at the beginning of the text segment or at the ELF entry point).
We plumb the result through to the global data structure.

Signed-off-by: Simon Glass <sjg@chromium.org>