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/cmd/pxe.c b/cmd/pxe.c
index 080b376..9434a18 100644
--- a/cmd/pxe.c
+++ b/cmd/pxe.c
@@ -571,7 +571,7 @@
 
 /*
  * Boot a label that specified 'localboot'. This requires that the 'localcmd'
- * environment variable is defined. Its contents will be executed as U-boot
+ * environment variable is defined. Its contents will be executed as U-Boot
  * command.  If the label specified an 'append' line, its contents will be
  * used to overwrite the contents of the 'bootargs' environment variable prior
  * to running 'localcmd'.
@@ -1438,7 +1438,7 @@
 }
 
 /*
- * Converts a pxe_menu struct into a menu struct for use with U-boot's generic
+ * Converts a pxe_menu struct into a menu struct for use with U-Boot's generic
  * menu code.
  */
 static struct menu *pxe_menu_to_menu(struct pxe_menu *cfg)