partuuids are represented as hexadecimal strings, where case doesn't
matter. barebox formats them as lower case internally, forcing the
partuuid device tree property to be lower case too. Use strcasecmp to
be case-insensitive.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>