base-files: add function to get mac as text from flash

Add a function to get a mac stored as text from flash. The octets of
the mac address need to be separated by any separator supported by
macaddr_canonicalize().

Signed-off-by: Dmitry Tunin <hanipouspilot@gmail.com>
Signed-off-by: Mathias Kresin <dev@kresin.me>
(backported from dfee452713)

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

@@ -38,6 +38,29 @@ mtd_get_mac_ascii() {
 	[ -n "$mac_dirty" ] && macaddr_canonicalize "$mac_dirty"
 }
 
+mtd_get_mac_text() {
+	local mtdname=$1
+	local offset=$2
+	local part
+	local mac_dirty
+
+	part=$(find_mtd_part "$mtdname")
+	if [ -z "$part" ]; then
+		echo "mtd_get_mac_text: partition $mtdname not found!" >&2
+		return
+	fi
+
+	if [ -z "$offset" ]; then
+		echo "mtd_get_mac_text: offset missing!" >&2
+		return
+	fi
+
+	mac_dirty=$(dd if="$part" bs=1 skip="$offset" count=17 2>/dev/null)
+
+	# "canonicalize" mac
+	[ -n "$mac_dirty" ] && macaddr_canonicalize "$mac_dirty"
+}
+
 mtd_get_mac_binary() {
 	local mtdname="$1"
 	local offset="$2"