Add MBEDTLS_TARGET_PREFIX to 3rdparty CMake

MBEDTLS_TARGET_PREFIX is prepended to the CMake targets for Mbed TLS
except for targets in 3rdparty. Change this so that 3rdparty targets use
the prefix as well.

This allows multiple copies of Mbed TLS to be used in the same CMake
tree when using code in the 3rdparty directory.

Signed-off-by: David Horstmann <david.horstmann@arm.com>
diff --git a/3rdparty/p256-m/CMakeLists.txt b/3rdparty/p256-m/CMakeLists.txt
index 0001dd2..7b6e13a 100644
--- a/3rdparty/p256-m/CMakeLists.txt
+++ b/3rdparty/p256-m/CMakeLists.txt
@@ -1,8 +1,10 @@
-add_library(p256m
+set(p256m_target ${MBEDTLS_TARGET_PREFIX}p256m)
+
+add_library(${p256m_target}
     p256-m_driver_entrypoints.c
     p256-m/p256-m.c)
 
-target_include_directories(p256m
+target_include_directories(${p256m_target}
   PUBLIC $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}>
          $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/p256-m>
          $<BUILD_INTERFACE:${MBEDTLS_DIR}/include>
@@ -19,7 +21,7 @@
 
 endif(INSTALL_MBEDTLS_HEADERS)
 
-install(TARGETS p256m
+install(TARGETS ${p256m_target}
 EXPORT MbedTLSTargets
 DESTINATION ${CMAKE_INSTALL_LIBDIR}
 PERMISSIONS OWNER_READ OWNER_WRITE GROUP_READ WORLD_READ)