barebox.bbclass: set KBUILD_OUTPUT

barebox.bbclass: set KBUILD_OUTPUT

Sets KBUILD_OUTPUT to the build directory to ensure using the valid
path in e.g. menuconfig or devshell.

An externally set KBUILD_OUTPUT could otherwise confuse the build
process and create the .config from an invalid folder.

This is what the kernel recipes also do.

Signed-off-by: Enrico Jörns <ejo@pengutronix.de>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>