mbed: Add CMake support and fix files paths

The mbed port should be compatible with a new command-line tool
mbed-tools (Mbed CLI 2) which use cmake build system. This allows build
cmake-base applications.
Fix mbed files paths - it's required for the new version of mbed-os
(v6.15)

Signed-off-by: Artur Tynecki <artur.tynecki@mobica.com>