pkg-config: add initial pkg-config files

Add three package config files for mbedtls, mbedcrypto and mbedx509.
Also update various project variables so the generated PC files have the
required data needed without hardcoding it everywhere.

This will help distros package the project following existing
conventsions between a normal and -devel package that includes the
headers and .pc files for pkg-config aware consumers.

Fixes: #228
Signed-off-by: Bill Roberts <bill.roberts@arm.com>

pkgconfig/JoinPaths.cmake

@@ -0,0 +1,27 @@
+# SPDX-License-Identifier: Apache-2.0 OR GPL-2.0-or-later
+# This module provides function for joining paths
+# known from most languages
+#
+# Copyright The Mbed TLS Contributors
+#
+# This script originates from:
+#   - https://github.com/jtojnar/cmake-snips
+# Jan has provided re-licensing under Apache 2.0 and GPL 2.0+ and
+# allowed for the change of Copyright.
+#
+# Modelled after Python’s os.path.join
+# https://docs.python.org/3.7/library/os.path.html#os.path.join
+# Windows not supported
+function(join_paths joined_path first_path_segment)
+    set(temp_path "${first_path_segment}")
+    foreach(current_segment IN LISTS ARGN)
+        if(NOT ("${current_segment}" STREQUAL ""))
+            if(IS_ABSOLUTE "${current_segment}")
+                set(temp_path "${current_segment}")
+            else()
+                set(temp_path "${temp_path}/${current_segment}")
+            endif()
+        endif()
+    endforeach()
+    set(${joined_path} "${temp_path}" PARENT_SCOPE)
+endfunction()