After the override syntax change, toaster isn't working correctly. This
is because it uses IMAGE_INSTALL_append instead of IMAGE_INSTALL:append.
This tweaks the code accordingly to fix this.
I have a suspicion that exiting toaster databases may struggle with
the change and there are some migration steps missing for the whole
overrides syntax change step.
(Bitbake rev: e0a450db6d5a50fe7682f62936dfa082bd5de121)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>