board/cubietech/cubieboard1: use lowercase keywords in extlinux.conf file

This commit also updates the label name, and the unnecessary "default"
entry is also removed.

Signed-off-by: Scott Fan <fancp2007@gmail.com>
Signed-off-by: Romain Naour <romain.naour@smile.fr>

board/cubietech/cubieboard1/rootfs_overlay/boot/extlinux/extlinux.conf

@@ -1,5 +1,4 @@
-DEFAULT linux
-LABEL linux
-  KERNEL /boot/zImage
-  DEVICETREE /boot/sun4i-a10-cubieboard.dtb
-  APPEND console=ttyS0,115200 earlyprintk root=/dev/mmcblk0p1 rootwait
+label cubieboard-buildroot
+  kernel /boot/zImage
+  devicetree /boot/sun4i-a10-cubieboard.dtb
+  append console=ttyS0,115200 earlyprintk root=/dev/mmcblk0p1 rootwait