CMake: Create a mbed-os alias library
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.

mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed