Use correct spelling of "U-Boot"

Correct spelling of "U-Boot" shall be used in all written text
(documentation, comments in source files etc.).

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Minkyu Kang <mk7.kang@samsung.com>
diff --git a/board/freescale/mx28evk/README b/board/freescale/mx28evk/README
index f9d6324..a248fb2 100644
--- a/board/freescale/mx28evk/README
+++ b/board/freescale/mx28evk/README
@@ -1,7 +1,7 @@
 FREESCALE MX28EVK
 ==================
 
-Supported hardware: MX28EVK rev C and D are supported in U-boot.
+Supported hardware: MX28EVK rev C and D are supported in U-Boot.
 
 Files of the MX28EVK port
 --------------------------