Create a .gitignore file in the CANONICAL_O directory

This .gitignore file ignores all files in an output directory and
hence doesn't change the git status.

Remove "/output" entry from the main .gitignore file as it is already
handled by this general approach.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Brandon Maier <brandon.maier@collins.com>
Signed-off-by: Arnout Vandecappelle <arnout@mind.be>

.gitignore

@@ -1,4 +1,3 @@
-/output
 /dl
 /.auto.deps
 /.config.cmd

Makefile

@@ -63,6 +63,9 @@ ifneq ($(findstring @,$(CANONICAL_O)),)
 $(error The build directory can not contain a '@')
 endif
 
+# Create a default .gitignore file that ignores everything
+$(shell echo "*" > "$(CANONICAL_O)/.gitignore")
+
 CANONICAL_CURDIR = $(realpath $(CURDIR))
 
 REQ_UMASK = 0022