[build] Avoid errors when build directory is mounted via NFS

Reported-by: Robin Smidsrød <robin@smidsrod.no>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/x86/Makefile.linux

 
 $(BIN)/%.linux : $(BIN)/%.linux.tmp
 	$(QM)$(ECHO) "  [FINISH] $@"
-	$(Q)cp -p $< $@
+	$(Q)$(CP) $< $@

src/util/geniso

 shift
 dir=`mktemp -d bin/iso.dir.XXXXXX`
 cfg=$dir/isolinux.cfg
-cp -p $isolinux_bin $dir
+cp $isolinux_bin $dir
 
 # syslinux 6.x needs a file called ldlinux.c32
 ldlinux_c32=$(dirname ${isolinux_bin})/ldlinux.c32
 if [ -s ${ldlinux_c32} ]
 then
-	cp -p ${ldlinux_c32} ${dir}
+	cp ${ldlinux_c32} ${dir}
 fi
 
 cat > $cfg <<EOF
 	first=$g
 	echo LABEL $b
 	echo "" KERNEL $g
-	cp -p $f $dir/$g
+	cp $f $dir/$g
 done >> $cfg
 $mkisofs -quiet -l -o $out -c boot.cat -b isolinux.bin -no-emul-boot -boot-load-size 4 -boot-info-table $dir
 rm -fr $dir