CMake: Create a mbed-os alias library

This ensures that applications which link with the mbed-os
CMake target still work. We want applications that require the OS
to use the mbed-os CMake target as we will be creating a new one
that will include mbed-core and mbed-rtos.

Author: hugueskamba

CMakeLists.txt

@@ -14,6 +14,8 @@ include(${MBED_ROOT}/tools/cmake/core.cmake)
 include(${MBED_ROOT}/tools/cmake/profile.cmake)
 
 add_library(mbed-core OBJECT)
+# TODO CMAKE: Replace mbed-os by a new one that will include mbed-core and mbed-rtos
+add_library(mbed-os ALIAS mbed-core)
 
 # Validate selected C library type
 # The C library type selected has to match the library that the target can support