CMake: support "make programs"

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/programs/CMakeLists.txt b/programs/CMakeLists.txt
index 0633aa6..aaf93ba 100644
--- a/programs/CMakeLists.txt
+++ b/programs/CMakeLists.txt
@@ -1,3 +1,6 @@
+set(programs_target "${MBEDTLS_TARGET_PREFIX}programs")
+add_custom_target(${programs_target})
+
 add_subdirectory(aes)
 add_subdirectory(cipher)
 if (NOT WIN32)