Add 'bom_item' field to BuildItem model

- Required to link the build to the output in case of variant stock

InvenTree/build/migrations/0028_builditem_bom_item.py

@@ -0,0 +1,20 @@
+# Generated by Django 3.2 on 2021-06-01 05:23
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('part', '0066_bomitem_allow_variants'),
+        ('build', '0027_auto_20210404_2016'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='builditem',
+            name='bom_item',
+            field=models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.CASCADE, related_name='allocate_build_items', to='part.bomitem'),
+        ),
+    ]