Merge remote-tracking branch 'origin/NR_feature_set_update' into integration_2021_wk37

doc/BUILD.md

@@ -77,7 +77,7 @@ cd cmake_targets/
 ./build_oai -I -w USRP --eNB --UE --nrUE --gNB
 ```
 
-- The `-I` option is to install pre-requisites, you only need it the first time you build the softmodem or when some oai dependencies have changed. 
+- The `-I` option is to install pre-requisites, you only need it the first time you build the softmodem or when some oai dependencies have changed. Note: for Ubuntu 20 use cmake_targets/install_external_packages.ubuntu20 instead!
 - The `-w` option is to select the radio head support you want to include in your build. Radio head support is provided via a shared library, which is called the "oai device" The build script creates a soft link from `liboai_device.so` to the true device which will be used at run-time (here the USRP one,`liboai_usrpdevif.so` . USRP is the only hardware tested today in the Continuous Integration process. The RF simulator[RF simulator](../targets/ARCH/rfsimulator/README.md) is implemented as a specific device replacing RF hardware, it can be specifically built using `-w SIMU` option, but is also built during any softmodem build.
 - `--eNB` is to build the `lte-softmodem` executable and all required shared libraries
 - `--gNB` is to build the `nr-softmodem` executable and all required shared libraries