mx6cuboxi: Mention Cubox-i in the README

Cubox-i boards are also supported, so update the README file.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>

board/solidrun/mx6cuboxi/README

-How to use U-boot on Solid-run mx6 hummingboard
------------------------------------------------
+How to use U-boot on Solid-run mx6 Hummingboard and Cubox-i
+-----------------------------------------------------------
 
-- Build U-boot for hummingboard:
+- Build U-boot for Hummingboard/Cubox-i:
 
 $ make mrproper
 $ make mx6cuboxi_defconfig
@@ -17,5 +17,5 @@ sudo dd if=SPL of=/dev/mmcblk0 bs=1k seek=1; sync
 
 sudo dd if=u-boot.img of=/dev/mmcblk0 bs=1k seek=69; sync
 
-- Insert the SD card in the hummingboard, power it up and U-boot messages
-should come up.
+- Insert the SD card in the board, power it up and U-boot messages should
+come up.