Build wic images with bitbake

Hi Reviewers,

This patchset adds new image type 'wic' to the list of image types supported by bitbake. This should allow to build partitioned image the same way as any other images.

New image type can be used in image recipes almost the same way as other image types. There is just one difference - <image>[.<machine>].wks file should be put to the same location as image recipe.

To enable this functionality I've implemented generating .env files with bitbake variables and reading them by wic. This seems to be the only way to get bitbake variables in wic when it's run by bitbake as running 'bitbake -e' is not possible as bitbake is locked.

Test cases for new functionality and small improvements are also included into this patchset.

Changes in V2:
- Fixed typo in the comment. Thanks Mario!
- Rebased on fresh master

The following changes since commit bb34181b42b62e0476ba9e12f9125ed9f785cf23:

bitbake: lib/bb/main: avoid importing all server/UI modules on every execution (2015-08-24 23:47:07 +0100)

are available in the git repository at:

git://git.yoctoproject.org/poky-contrib ed/wic/wic-image-type
http://git.yoctoproject.org/cgit.cgi/poky-contrib/log/?h=ed/wic/wic-image-type

Ed Bartosh (17):
image.py: write bitbake variables to .env file
oe-selftest: test generation of <image>.env
wic: remove undescore from function name
wic: add BitbakeVars class
wic: create new method _parse_line
wic: add default_image attribute to BitbakeVars
wic: set default image
wic: implement getting variables from .env files
wic: implement --vars option
wic: rename variable
wic: deferred call of hlp.get_wic_plugins_help() image_types.bbclass: add wic image type
wic-image-minimal: add wic image recipe and .wks
oe-selftest: test building wic image by bitbake image.py: set bitbake variable ROOTFS_SIZE
wic: use bitbake variable ROOTFS_SIZE image.py: add script output to the rootfs log

 meta/classes/image_types.bbclass                   |  12 ++
 meta/lib/oe/image.py                               |  32 +++++-
 meta/lib/oeqa/selftest/wic.py                      |  33 ++++++
 meta/recipes-extended/images/wic-image-minimal.bb  |  14 +++
 meta/recipes-extended/images/wic-image-minimal.wks |  10 ++
 scripts/lib/image/help.py                          |   6 +-
 scripts/lib/wic/imager/direct.py                   |  14 +++
 scripts/lib/wic/utils/oe/misc.py                   | 125 ++++++++++++++-------
 scripts/wic                                        |  14 ++-
 9 files changed, 212 insertions(+), 48 deletions(-)
 create mode 100644 meta/recipes-extended/images/wic-image-minimal.bb
 create mode 100644 meta/recipes-extended/images/wic-image-minimal.wks

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Regards,
Ed