bitbake.conf/base.bbclass: Rename the staging directory to sysroots and the populate_staging task to populate_sysroot
This change makes the purpose of the staging directory more obvious and
the taskname more true to what it now actually does.
The layout version number is increased due to the change in layout
but code to convert existing directories and insert a symlink for
backwards compatibility is included.
Signed-off-by: Richard Purdie <rpurdie@linux.intel.com>
