Install CMake files in MbedTLS dir

Right now, CMake files are installed in <prefix>/cmake. That being said,
it gets easily bloated, and the standard is to use a directory with the
same name as the project.

I discovered this issue with this "bug":
https://github.com/termux/termux-packages/issues/12416
The issue's author claimed that MbedTLS's files were not installed in
the lib directory. But the patch applied by termux team broke CMake's
search of MbedTLS config files. So I wanted to upstream the real fix
here instead.

Here are some examples of projects using directories:
 - https://github.com/xiph/flac/blob/1.4.2/CMakeLists.txt#L239
 - https://gitlab.freedesktop.org/dbus/dbus/-/blob/dbus-1.15.2/CMakeLists.txt#L675
 - https://github.com/catchorg/Catch2/blob/v3.2.0/CMakeLists.txt#L62
 - https://github.com/capnproto/capnproto/blob/v0.10.2/c++/CMakeLists.txt#L162

Signed-off-by: Issam E. Maghni <issam.e.maghni@mailbox.org>
diff --git a/ChangeLog.d/cmake-install.txt b/ChangeLog.d/cmake-install.txt
new file mode 100644
index 0000000..d8eb72e
--- /dev/null
+++ b/ChangeLog.d/cmake-install.txt
@@ -0,0 +1,3 @@
+Changes
+  * Install the .cmake files into CMAKE_INSTALL_LIBDIR/cmake/MbedTLS,
+    typically /usr/lib/cmake/MbedTLS.