wic: Add OpenEmbedded-specific implementation

wic: Add OpenEmbedded-specific implementation

Reuses the mic/livecd infrastructure but heavily subclasses and
modifies it to adapt to the special needs of building images from
existing OpenEmbedded build artifacts.

In addition to the OE-specific mic objects and modifications to the
underlying infrastructure, this adds a mechanism to allow OE kickstart
files to be 'canned' and made available to users via the 'wic list
images' command.

Two initial OE kickstart files have been added as canned .wks files:
directdisk, which implements the same thing as the images created by
directdisk.bbclass, and mkefidisk, which can essentially be used as a
replacement for mkefidisk.sh.  Of course, since creation of these
images are now driven by .wks files rather than being hard-coded into
class files or scripts, they can be easily modified to generate
different variations on those images.  They also don't require root
priveleges, since they don't use mount to create the images.  They
don't however write to media like mkefidisk.sh does, but rather create
images that can be written onto media.

Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>