bootloader: move esp32 specific code from bootloader_common

Also implements bootloader_clock_get_rated_freq_mhz for esp32s2beta.

Closes IDF-758

components/bootloader_support/src/esp32/bootloader_esp32.c

@@ -1,3 +1,17 @@
+// Copyright 2019 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.
+
 #include "bootloader_common.h"
 #include "sdkconfig.h"
 #include "soc/efuse_reg.h"