gen_links.sh: auto-generate .gitignore files to ignore symlinks

In each subdir that is a target for symlinks, automatically ignore the results
of gen_links():

- At the top of gen_links.sh.inc, clear the .gitignore.
- In the loop, add each link name to the local .gitignore.
- In selected gen_links.sh, there is also a "manual" link creationg. So that
  this also ends up in the local .gitignore, have the link creation as separate
  gen_link() macro which at the same time adds to ./.gitignore.
- in the root .gitignore, ignore all the subdirs' generated */.gitignore files.

Change-Id: I73c11fe8362358bf7e1bdf0e1be53399b5d3351b
diff --git a/.gitignore b/.gitignore
index d8c3ab3..cc06c3c 100644
--- a/.gitignore
+++ b/.gitignore
@@ -5,3 +5,4 @@
 *.log
 *.so
 compile
+*/.gitignore