scripts/dtc: Add missing fdtoverlay to gitignore

Commit 0da6bcd9 ("scripts: dtc: Build fdtoverlay tool") enabled
building fdtoverlay, but failed to add it to .gitignore.

Also add a note to keep hostprogs in sync with .gitignore.

Fixes: 0da6bcd9

 ("scripts: dtc: Build fdtoverlay tool")
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>