.gitignore: ignore generated u-boot.imx

Signed-off-by: Stefano Babic <sbabic@denx.de>

.gitignore

@@ -21,6 +21,7 @@
 /System.map
 /u-boot
 /u-boot.hex
+/u-boot.imx
 /u-boot.map
 /u-boot.bin
 /u-boot.srec