.gitignore: ignore boot images by pattern rule /u-boot*

U-Boot supports various boot images for various SoCs.
It is annoying to modify .gitignore file every time
we add/delete boot images.

Fortunately, there is a simple rule:
Those with file name prefix "u-boot" at the top directory
are all generated files.
Signed-off-by: NMasahiro Yamada <yamada.m@jp.panasonic.com>

.gitignore

@@ -31,24 +31,7 @@
 /MLO*
 /SPL
 /System.map
-/u-boot
-/u-boot.hex
-/u-boot.imx
-/u-boot-with-spl.imx
-/u-boot-with-nand-spl.imx
-/u-boot.map
-/u-boot.srec
-/u-boot.ldr
-/u-boot.ldr.hex
-/u-boot.ldr.srec
-/u-boot.img
-/u-boot.kwb
-/u-boot.sha1
-/u-boot.dis
-/u-boot.lds
-/u-boot.ubl
-/u-boot.ais
-/u-boot.sb
+/u-boot*
 
 #
 # git files that we don't want to ignore even it they are dot-files