mkisoimages.sh: don't use -p flag when copying loader.efi to msdosfs.

This fixes 'cdrom' target in the case when world was built by user,
and not root.

Reviewed by:	imp
Differential revision:	https://reviews.freebsd.org/D18414

release/amd64/mkisoimages.sh

@@ -49,7 +49,7 @@ if [ "$1" = "-b" ]; then
 	mkdir efi
 	mount -t msdosfs /dev/$device efi
 	mkdir -p efi/efi/boot
-	cp -p "$BASEBITSDIR/boot/loader.efi" efi/efi/boot/bootx64.efi
+	cp "$BASEBITSDIR/boot/loader.efi" efi/efi/boot/bootx64.efi
 	umount efi
 	rmdir efi
 	mdconfig -d -u $device