$(CP) was defined as `cp -fpR' since the very begining of OpenWrt build
system (2006-06-22).  The -R option should be enough and base packages
use only $(CP) for the same purposes just fine and BSD manual of cp also
discourages the use of `-r' option.  So let's just tidy up the usage now.

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>