Remove improper uImage file

Remove boards/arm/sama5/sama5d3-xplained/boot/uImage that can in inadvertantly with a PR merge.

Also, change "/uImage" to just "uImage" in the top-level .gitignore file so that all occurences of uImage will be ignored, not just versions in the nuttx/ top-level directory.

.gitignore

@@ -8,6 +8,7 @@ Make.dep
 .swp
 .*.swp
 core
+uImage
 /.gdbinit
 /cscope.*
 /.config
@@ -21,7 +22,6 @@ core
 /nuttx
 /nuttx.*
 /nuttx-*
-/uImage
 /_SAVED_APPS_config
 /*.map
 /*.elf