Added derivative build support to RHEL7

Author: Martin Preisler

RHEL/7/CMakeLists.txt

@@ -1 +1,8 @@
 ssg_build_product("rhel7")
+
+if (SSG_CENTOS_DERIVATIVES_ENABLED)
+    ssg_build_derivative_product("rhel7" "centos" "centos7")
+endif()
+if (SSG_SCIENTIFIC_LINUX_DERIVATIVES_ENABLED)
+    ssg_build_derivative_product("rhel7" "sl" "sl7")
+endif()