Fixed the pacing logic in bootchart data collection.

Ideally bootchart collects system data every BOOTCHART_POLLING_MS ms.
However, the current bootchart pacing logic would collect data every
time the main loop in init.c is executed. This results in:

1. Multiple data samples being taken consecutively or prematurely. In
many cases, these data samples have the same timestamp (in jiffies).
The off-line data processing tool would complain about dividing by
zero "interval" and bail out.

2. Because of the ineffective pacing, bootchart data collection would
terminate prematurely. The total duration of data collection is usually
much shorter than what the user specifies.

The fix is to check whether BOOTCHART_POLLING_MS ms has elapsed
before taking a new data sample. For this purpose, /proc/uptime
is used to get the time values, consistent with the precision of
bootchart timestamps.

Change-Id: I106bf91dbda01059b719df6c73b8bd1cd54a64f0
Signed-off-by: Bo (Andover) Zhang <zhang@broadcom.com>