[Toaster] [RFC] Handling IMAGE_INSTALL_append

[Barros Pena, Belen]

The default project configuration we ship with Toaster includes the
IMAGE_INSTALL_append variable. This variable requires a space before the
first package name. We have spoken before about taking 2 possible
approaches to handle this:

1. Toaster takes responsibility and adds the space for you. How this could
be done, I am not sure. I guess we can either somehow identify
IMAGE_INSTALL_append as the type of variable that requires a space before
the first list item, or we can hardcode it.

2. Users take responsibility and add the space themselves. Easier to
implement (we add some help text to the field), but bound to fail (users
don't read help text).

>From my perspective, option 1 is the best one. I don't care if we need to
hardcode it. Adding this variable to the default project configuration is
very much a temporary solution until we provide functionality to customise
the list of packages installed by an image recipe.

Would this be acceptable to everybody else?

Thanks!

Belén