bootstage: Implement core microsecond boot time measurement

bootstage: Implement core microsecond boot time measurement

This defines the basics of a new boot time measurement feature. This allows
logging of very accurate time measurements as the boot proceeds, by using
an available microsecond counter.

To enable the feature, define CONFIG_BOOTSTAGE in your board config file.
Also available is CONFIG_BOOTSTAGE_REPORT which will cause a report to be
printed just before handing off to the OS.

Most IDs are not named at this stage. For that I would first like to
renumber them all.

Timer summary in microseconds:
       Mark    Elapsed  Stage
          0          0  reset
    205,000    205,000  board_init_f
  6,053,000  5,848,000  bootm_start
  6,053,000          0  id=1
  6,058,000      5,000  id=101
  6,058,000          0  id=100
  6,061,000      3,000  id=103
  6,064,000      3,000  id=104
  6,093,000     29,000  id=107
  6,093,000          0  id=106
  6,093,000          0  id=105
  6,093,000          0  id=108
  7,089,000    996,000  id=7
  7,089,000          0  id=15
  7,089,000          0  id=8
  7,097,000      8,000  start_kernel

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