.gitignore: ignore files generated by Toaster

When Toaster is run as documented on
https://docs.yoctoproject.org/toaster-manual/setup-and-use.html



Signed-off-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>