[OE-core] Speed up sdk creation by supporting multicore compressors.

[Randy Witt]

This change allows the user to pick a different compressor to be used after the
sdk is ran through tar. In my testing I found it speeds up sdk creation times
~20-30% if a compressor that supports multiple cores is used. Here are some
measurments:

Using default compressor(bzip2): core-image-minimal-1.0-r0: do_populate_sdk:
Elapsed time: 133.21 seconds CPU usage: 12.3% core-image-sato-1.0-r0:
do_populate_sdk: Elapsed time: 356.85 seconds CPU usage: 11.6%

Using pigz: core-image-minimal-1.0-r0: do_populate_sdk: Elapsed time: 102.62
seconds CPU usage: 15.3% core-image-sato-1.0-r0: do_populate_sdk: Elapsed time:
261.66 seconds CPU usage: 15.6%


The default compressor hasn't been changed and so I've just been doing it using
local.conf for now, but if desired I can submit another patch changing
SDK_COMPRESSION_TYPE to pigz by default.

This also allows for easy use of xz when disk space concerns are more important
than speed.