AK2: fix patch_fstab substring replacement

- the 'original string' ($5) was not being used, instead the entire part was always being replaced with the 'replacement string' ($6)
- replace the target 'original string' of the entry part with 'replacement string' as expected/intended

NOTE: any properly written patch_fstab commands with original string as the complete original part WILL remain working as this is a backwards compatible change

tools/ak2-core.sh

@@ -364,7 +364,8 @@ patch_fstab() {
       options) part=$(echo "$entry" | awk '{ print $4 }');;
       flags) part=$(echo "$entry" | awk '{ print $5 }');;
     esac;
-    newentry=$(echo "$entry" | sed "s;${part};${6};");
+    newpart=$(echo "$part" | sed "s;${5};${6};");
+    newentry=$(echo "$entry" | sed "s;${part};${newpart};");
     sed -i "s;${entry};${newentry};" $1;
   fi;
 }