RFC skipped recipe handling

I noticed bitbake-layers did not handle bbappends for skipped recipes properly (e.g. in Poky it complained that uclibc_git.bbappend did not match any recipe). Thus I added a mechanism to bitbake within cooker that lists skipped recipes and extended bitbake-layers to use it.

This (or something similar) will also be needed for improving the NoProvider error handling - ideally we should be reporting if the target being requested has been skipped and why. To this end the SkippedPackage class includes the reason and DEPENDS/RDEPENDS info (although at the moment RDEPENDS does not get populated). Some may prefer I remove the DEPENDS/RDEPENDS bits until the future patch where they are used, if so I wouldn't object to removing them.

I also considered adding to skiplist for BBMASKed-out recipes but on reflection I elected not to do so since I assumed we ought to be completely ignoring recipes that are masked out in this way.

Patch summary (these are against Poky but they apply cleanly with -p2 against bitbake master):

Paul Eggleton (2):
bitbake: track skipped packages
bitbake-layers: handle skipped recipes

 bitbake/bin/bitbake-layers  |   24 +++++++++++++++++++-----
 bitbake/lib/bb/cache.py     |   12 +++++++++---
 bitbake/lib/bb/cooker.py    |   16 ++++++++++++++++
 bitbake/lib/bb/parse/ast.py |   12 ++++++------
 4 files changed, 50 insertions(+), 14 deletions(-)

1.7.4.1