classes/populate_sdk_ext: show progress when preparing build system

classes/populate_sdk_ext: show progress when preparing build system

During the extensible SDK installation process the final step is to
prepare the internal copy of the build system. This can take some time,
especially if you have SDK_EXT_TYPE set to "minimal" (downloading
sstate artifacts) and SDK_INCLUDE_PKGDATA set to "1" (restoring
pkgdata for world). To make this a bit less painful, use BitBake's new
quiet mode to display status during this operation so you have some idea
of how it's progressing; instead of redirecting the output to
preparing_build_system.log we grab the last console log and append it
instead.

One result of this change is that you get the errors printed on the
console during normal output rather than this going to the
preparing_build_system.log file first. In OE-Core revision
227d2cbf9e0b8c35fa6644e3d72e0699db9607fa, we changed to always print the
contents of preparing_build_system.log on failure, but now at least the
error contents of that log is duplicated. Besides, I intentionally
didn't print out the contents of that log during normal usage because
it's quite verbose - the bug that we were attempting to fix was about
not getting this information when seeing failures in the automated
tests, thus I've moved printing the log to the test handling code
instead.

Part of the implementation for [YOCTO #9613].

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>