[script] perserving the permission when cp openthread files (#95)

In some environments, `cp` may produce files which have different
permissions from the source files. This can fail the build of
OTBR. Hence we need to add `-p` option in the command to preserve the
permissions.

script/make-firmware.bash

@@ -193,7 +193,7 @@ build_ot()
             rm -rf openthread
             # git_archive_all doesn't accept symbolic link, so make a copy of openthread and make
             # it not a submodule
-            cp -r ../openthread .
+            cp -rp ../openthread .
             rm openthread/.git
 
             # Build

script/otbr-setup.bash

@@ -191,7 +191,7 @@ apt-get install -y --no-install-recommends git python3-pip
 su -c "DOCKER=1 ${build_options[*]} script/bootstrap" pi
 
 rm -rf /home/pi/repo/ot-br-posix/third_party/openthread/repo
-cp -r /home/pi/repo/openthread /home/pi/repo/ot-br-posix/third_party/openthread/repo
+cp -rp /home/pi/repo/openthread /home/pi/repo/ot-br-posix/third_party/openthread/repo
 
 apt-get purge -y cmake
 pip3 install scikit-build