Update CMake minimum version required from 3.18.2 to 3.19.0

Replace mbed-os source path MBED_ROOT with MBED_PATH in CMakeLists.txt

Author: rajkan01

CMakeLists.txt

@@ -1,16 +1,15 @@
 # Copyright (c) 2020 ARM Limited. All rights reserved.
 # SPDX-License-Identifier: Apache-2.0
 
-cmake_minimum_required(VERSION 3.18.2 FATAL_ERROR)
+cmake_minimum_required(VERSION 3.19.0 FATAL_ERROR)
 
-# TODO: @mbed-os-tools MBED_ROOT and MBED_CONFIG_PATH should probably come from mbedtools
-set(MBED_ROOT ${CMAKE_CURRENT_SOURCE_DIR}/mbed-os CACHE INTERNAL "")
+set(MBED_PATH ${CMAKE_CURRENT_SOURCE_DIR}/mbed-os CACHE INTERNAL "")
 set(MBED_CONFIG_PATH ${CMAKE_CURRENT_SOURCE_DIR}/.mbedbuild CACHE INTERNAL "")
 set(APP_TARGET mbed-os-example-devicekey)
 
-include(${MBED_ROOT}/tools/cmake/app.cmake)
+include(${MBED_PATH}/tools/cmake/app.cmake)
 
-add_subdirectory(${MBED_ROOT})
+add_subdirectory(${MBED_PATH})
 
 add_executable(${APP_TARGET})