uboot-extlinux-config.bbclass: fix old override syntax in comment

uboot-extlinux-config.bbclass: fix old override syntax in comment

The comment specifies how to use the variables but uses the older and
now unsupported override syntax. Let's update to match the newer syntax.

Cc: Quentin Schulz <foss+yocto@0leil.net>
(From OE-Core rev: 0a381eea4d50ff1c6e7c7d0d4df62eb581454b48)

Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit bb64f3fed29b9532e6ddc9a2ba0283d373622d87)
Signed-off-by: Steve Sakoman <steve@sakoman.com>