base-files: cleanup mtd_get_mac_binary hexdump

Remove usage of dd, as per request from Yousong Zhou.
<https://lists.openwrt.org/pipermail/openwrt-devel/2015-December/037743.html

>

Signed-off-by: Chris R Blake <chrisrblake93@gmail.com>

SVN-Revision: 47949

package/base-files/files/lib/functions/system.sh

@@ -38,7 +38,7 @@ mtd_get_mac_binary() {
 		return
 	fi
 
-	dd bs=1 skip=$offset count=6 if=$part 2>/dev/null | hexdump -v -n 6 -e '5/1 "%02x:" 1/1 "%02x"'
+	hexdump -v -n 6 -s $offset -e '5/1 "%02x:" 1/1 "%02x"' $part 2>/dev/null
 }
 
 mtd_get_mac_binary_ubi() {