Introduce <endian.h> compatibility header

Programs and libraries using compiler and system information about
endianness often include the system header `<endian.h>`.

In `xtensa-gcc` compiler with `newlib` distribution, that file is located in
`<machine/endian.h>`; this means that `#include <endian.h>` would fail
at compile time.

This commit fixes the issue by adding a compatibility `<endian.h>`
header which in turn just includes `<machine/endian.h>`.

Signed-off-by: Francesco Giancane <francesco.giancane@accenture.com>
Merges https://github.com/espressif/esp-idf/pull/4784

components/newlib/platform_include/endian.h

@@ -0,0 +1,25 @@
+// Copyright 2018 Espressif Systems (Shanghai) PTE LTD
+// Copyright 2020 Francesco Giancane <francesco.giancane@accenture.com>
+//
+// 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.
+
+/*
+ * This is a compatibility header for <endian.h>.
+ * In xtensa-newlib distribution it is located in <machine/endian.h>
+ * but most program expect to be plain <endian.h>.
+ */
+
+#ifndef XTENSA_COMPAT_ENDIAN_H_INCLUDED
+#define XTENSA_COMPAT_ENDIAN_H_INCLUDED
+#include <machine/endian.h>
+#endif /* XTENSA_COMPAT_ENDIAN_H_INCLUDED */