Fix image parsing when the upgrade file identifier is not at position 0 (#59)

lib/ota.js

@@ -8,7 +8,10 @@ function parseSubElement(buffer, position) {
     return {tagID, length, data};
 }
 
-function parseImage(buffer) {
+function parseImage(rawBuffer) {
+    const start = rawBuffer.indexOf(upgradeFileIdentifier);
+    const buffer = rawBuffer.slice(start);
+
     const header = {
         otaUpgradeFileIdentifier: buffer.subarray(0, 4),
         otaHeaderVersion: buffer.readUInt16LE(4),
@@ -55,4 +58,4 @@ function parseImage(buffer) {
 
 module.exports = {
     parseImage
-};
+};