buildstats.bbclass: Adding disk io measurement

This commit adds disk-io statistics functionality on a build
and per-task basis.

It pulls measurements for whatever partition TMPDIR exists on.
This data could be off if SSTATE_DIR and DL_DIR exist on
a different partition/volume.

Notes on what this pulls:

ReadsComp: Total number of reads complete
ReadsMerged: Total number of adjacent reads merged
SectRead: Total number of sectors read
TimeReads: Total number of m/s spent reading
WritesComp: Total number of writes completed
SectWrite: Total number of sectors written
TimeWrite: Total number of m/s spent writing
IOinProgress: Total amount of IO in progress at the time of
              we look at /proc/diskstats.
TimeIO: Total number of m/s spent doing IO
WTimeIO: Weighted time doing I/O. From iostats.txt:

"This field is incremented at each I/O start, I/O completion, I/O
 merge, or read of these stats by the number of I/Os in progress
 (field 9) times the number of milliseconds spent doing I/O since the
 last update of this field.  This can provide an easy measure of both
 I/O completion time and the backlog that may be accumulating."

Signed-off-by: Beth Flanagan <elizabeth.flanagan@intel.com>

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