Remove base_dir property on ramdisk filesystems

They shouldn't have the base directory in order to place init and the
system/ folder in the root of the ramdisk.

Bug: 379190521
Test: diff'd make and soong ramdisks, they're identical
Change-Id: I726c939c0f7d91a586e428affd1c5345dba4f64a
diff --git a/fsgen/filesystem_creator.go b/fsgen/filesystem_creator.go
index 6eae23a..a019087 100644
--- a/fsgen/filesystem_creator.go
+++ b/fsgen/filesystem_creator.go
@@ -598,7 +598,9 @@
 
 	fsProps.Partition_name = proptools.StringPtr(partitionType)
 
-	fsProps.Base_dir = proptools.StringPtr(partitionType)
+	if !strings.Contains(partitionType, "ramdisk") {
+		fsProps.Base_dir = proptools.StringPtr(partitionType)
+	}
 
 	fsProps.Is_auto_generated = proptools.BoolPtr(true)