toaster: fix help texts not showing for most tasks

These were not being collected properly because we were explicitly
excluding variables defined as functions from being stored in the
database. We don't want these to be shown in the variables list, and in
any case it makes sense for these to be stored elsewhere, so create a
separate model to store these.

Fixes [YOCTO #6050].

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
toaster: fix mis-detection of targets as images

If you are using the testimage class then the old test mistook the
global inheritance of testimage.bbclass with the recipe inheriting
image.bbclass because it was only looking for that at the end of the
string.

Also tidy up the code so you an easily tell what it's doing. (The
original method may have been more "pythonic", but it does nothing for
readability.)

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