WIP toaster updates

Recently, a bug was filed on the crops/toaster-container [1] which led me
to file a bug on bitbake [2] while also looking into updates to toaster
[3]. There is still an old reference to the “build mode” from the old docs
they needs to be cleaned up [4].

[1] https://github.com/crops/toaster-container/issues/32
[2] https://bugzilla.yoctoproject.org/show_bug.cgi?id=14753
[3]
https://git.yoctoproject.org/poky-contrib/log/?h=timo/toaster-fixes
[4]
https://git.yoctoproject.org/poky/tree/bitbake/lib/toaster/toastergui/templates/landing_not_managed.html#n34