Use libfdt for is-compatible checks

Testing the 'compatible' property of manifest FDT requires parsing a
list of strings. As this is currently the only use case, we can remove
a significant amount of code from the manifest parser by using a libfdt
API instead.

Change-Id: Idcfa8d9a85745738a08d4fdaa40f30cc18e3c5c2
diff --git a/src/manifest_test.cc b/src/manifest_test.cc
index 8273ae4..bed1675 100644
--- a/src/manifest_test.cc
+++ b/src/manifest_test.cc
@@ -304,8 +304,7 @@
 		.Build();
 	/* clang-format on */
 
-	ASSERT_EQ(manifest_from_vec(&m, dtb),
-		  MANIFEST_ERROR_PROPERTY_NOT_FOUND);
+	ASSERT_EQ(manifest_from_vec(&m, dtb), MANIFEST_ERROR_NOT_COMPATIBLE);
 }
 
 TEST(manifest, not_compatible)