.gitignore: Ignore .log files

The generated flash.log file that mach-imx uses for MKIMAGEOUTPUT is not
covered by any other ignore pattern. Ignore all *.log files.
Signed-off-by: NBaruch Siach <baruch@tkos.co.il>
Reviewed-by: NFabio Estevam <festevam@gmail.com>

.gitignore

@@ -41,6 +41,7 @@ fit-dtb.blob
 /System.map
 /u-boot*
 /boards.cfg
+/*.log
 
 #
 # git files that we don't want to ignore even it they are dot-files