image_types: Improve dependency handling between types (and use for sum.jffs2)

We're seeing a pattern of one image type needing to depend on another
type. A good example is jffs2 and sum.jffs2. This patch makes sum.jffs2
depend on jffs2 which will then allow a EXTRA_IMGAGECMD to be set for
sum.jffs2 individually without changing the jffs2 command. This allows the
-pad option to be configured differently.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
documentation.conf: update contents

This patch updates descriptions and brings more
info for the variable configurations as documented
in the OE-Core manual.

This file is used by Toaster to display help for
the configuration variables.

Signed-off-by: Alexandru DAMIAN <alexandru.damian@intel.com>
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
toaster: add class to dump toaster-tracked data

Adding a new bbclass that will collect and send relevant
data from the task context to the Toaster UI.

This bbclass consists of postfuncs that get executed
right after the main task func, and in the same context.
This allows data gathering in a synchronous manner during
the build, guaranteeing data integrity. This approach also
preserves the task signatures.

The data is moved to the UI through the event system.

There is no performance impact if the class is disabled.

License is MIT.

Signed-off-by: Alexandru DAMIAN <alexandru.damian@intel.com>
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>