lib/oe: Add sstatesig, OE specific signature generator classes
This patch adds SignatureGenerator classes specific to OE. For now,
these emulate the previous behaviour with the exception that
dependencies on quilt-native are now ignored for checksum purposes.
The intent is to allow easier experimentation and customisation of
this code in future as a result of these changes.
Note that these changes require pending bitbake patches.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
