.gitignore: Ignore regenerated *.dtbo files

*.dtbo are dt overlays files which should be also ignored as *.dtb.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>

.gitignore

@@ -10,6 +10,7 @@
 *.bin
 *.cfgout
 *.dtb
+*.dtbo
 *.dtb.S
 *.elf
 *.exe