Add time measurement command to u-boot

The command "time" takes a command as its argument, executes the command, and reports how much time the command has used.

Example usage:
CrOS> time sleep 1

time: 1.000 seconds, 1000 ticks
CrOS> time sleep 10

time: 10.000 seconds, 10000 ticks
TEST=see above

Review URL:

Change-Id: I97cb234c68f29e5db1f5a66abbe32ec6e575b847
3 files changed