bootstage: Create an initial header for boot progress integers

At present boot_stage_progress() is called with various magic numbers. The
new bootstage.h header will be used to turn these into symbolic names
throughout the code.

The intent is not that these numbers are passed to Linux. In fact by using
an enum to track them we should eventually be able to remove the explict
numbers and just have the stages count up from 0.

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