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>