bootstage: Add time accumulation feature

Sometimes we want to add up the amount of time spent in a particular
activity when it is happening in a number of discrete chunks.

Add bootstage_start() to mark the start of an acitivity and
bootstage_accum() to accumulate the time since the last start. Calling
these function in pairs results in the accumulated time being collected.

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