Carry MBedTLS patch that works around CMake 3.18.2 bug.

MBedTLS fails to build with CMake 3.18.2, due to a bug in
CMake 3.18.2 causing failure to find python3. For more information,
please see Mbed-TLS/mbedtls#3690 and
https://gitlab.kitware.com/cmake/cmake/-/issues/21204.

This pull request applies the MBedTLS patch that works around this
issue from Mbed-TLS/mbedtls#3691, which
has been merged into MBedTLS's development branch but not
yet into a tagged release.

Author: Sacha0

deps/mbedtls.mk

@@ -23,6 +23,8 @@ $(SRCCACHE)/$(MBEDTLS_SRC)/source-extracted: $(SRCCACHE)/$(MBEDTLS_SRC).tar.gz
 	$(JLCHECKSUM) $<
 	mkdir -p $(dir $@) && \
 	$(TAR) -C $(dir $@) --strip-components 1 -xf $<
+	# Apply workaround for CMake 3.18.2 bug (https://github.com/ARMmbed/mbedtls/pull/3691).
+	cd $(SRCCACHE)/$(MBEDTLS_SRC) && patch -p1 -f < $(SRCDIR)/patches/mbedtls-cmake-findpy.patch
 	# Force-enable MD4
 	sed "s|//#define MBEDTLS_MD4_C|#define MBEDTLS_MD4_C|" -i $(SRCCACHE)/$(MBEDTLS_SRC)/include/mbedtls/config.h
 	touch -c $(SRCCACHE)/$(MBEDTLS_SRC)/CMakeLists.txt # old target

deps/patches/mbedtls-cmake-findpy.patch

@@ -0,0 +1,40 @@
+From b250ac951dfd77ad7a128180fc8514ef5151cb5c Mon Sep 17 00:00:00 2001
+From: Paul Elliott <[email protected]>
+Date: Fri, 18 Sep 2020 18:15:07 +0100
+Subject: [PATCH] Set CMake policies for newer versions of CMake
+
+Make sure that CMP0012 is set to NEW, without which FindPython3 and
+FindPython2 functionality becomes broken with CMake 3.18.2 if searching
+by location. CMP0012 being set to OLD is also deprecated as of 3.18.3.
+Ensure CMP0011 is set to NEW to avoid warnings and deprecated behaviour
+being issued about policy push / pop with CMake 3.18.0 or newer.
+
+Signed-off-by: Paul Elliott <[email protected]>
+---
+ CMakeLists.txt | 12 ++++++++++++
+ 1 file changed, 12 insertions(+)
+
+diff --git a/CMakeLists.txt b/CMakeLists.txt
+index ac24bf41b4c..fdaa2f134d6 100644
+--- a/CMakeLists.txt
++++ b/CMakeLists.txt
+@@ -21,6 +21,18 @@
+ #
+ 
+ cmake_minimum_required(VERSION 2.8.12)
++
++# https://cmake.org/cmake/help/latest/policy/CMP0011.html
++# Setting this policy is required in CMake >= 3.18.0, otherwise a warning is generated. The OLD
++# policy setting is deprecated, and will be removed in future versions.
++cmake_policy(SET CMP0011 NEW)
++# https://cmake.org/cmake/help/latest/policy/CMP0012.html
++# Setting the CMP0012 policy to NEW is required for FindPython3 to work with CMake 3.18.2
++# (there is a bug in this particular version), otherwise, setting the CMP0012 policy is required
++# for CMake versions >= 3.18.3 otherwise a deprecated warning is generated. The OLD policy setting
++# is deprecated and will be removed in future versions.
++cmake_policy(SET CMP0012 NEW)
++
+ if(TEST_CPP)
+     project("mbed TLS" C CXX)
+ else()
+