Makefile: change rsync used in overlays to always transfer files

If two files with the same relative paths exist in multiple overlay
skeletons, and they have the same modification time and size, then rsync
might not copy the later file on top of the earlier file.  This patch fixes
this by adding the -I option to the rsync commands used in the overlay
skeleton file installations.  ("man rsync" indicates that this option turns
off the file-size/mod-date "quick check" behavior, causing all files to be
updated - more like the cp commands that we had originally.)

[Peter: use --ignore-times to make it obvious what the option does]
Signed-off-by: Danomi Manchego <danomimanchego123@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>