esp32: Fix backwards compatibility for pre-v2.1 bootloaders

Older bootloaders don't set RTC_XTAL_FREQ_REG or call rtc_clk_init(),
app needs to pick this up.

Reported at
https://esp32.com/viewtopic.php?f=2&t=3939&p=17836

components/bootloader_support/include/bootloader_clock.h

@@ -0,0 +1,21 @@
+// Copyright 2017 Espressif Systems (Shanghai) PTE LTD
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#pragma once
+
+/** @brief Configure clocks for early boot
+ *
+ * Called by bootloader, or by the app if the bootloader version is old (pre v2.1).
+ */
+void bootloader_clock_configure(void);