Revert "Switch "system/core/base" ref to "system/libbase""

This reverts commit e960b47673416a3af5ad5e012690fc27317a5a3a.

Reason for revert: Looks to have broken sc-d1-dev b/171770441

Change-Id: I958a95c3b57e46cab2ce17c4a4f9e80b45f2ad71
diff --git a/base b/base
new file mode 120000
index 0000000..622c552
--- /dev/null
+++ b/base
@@ -0,0 +1 @@
+../libbase
\ No newline at end of file