CMake: Replace `mbed_config_*.cmake` files with single `mdh_config.h` file

Historically, `mbed_config_*.cmake` files were toolchain specific. They have since evolved to not contain any toolchain specific definitions as toolchain
information is provided via the CMake configure command.
This commit removes `mbed_config_gcc.cmake` and `mbed_config_arm.cmake`
and replace them with a single `mdh_config.cmake` file.

Signed-off-by: Hugues Kamba-Mpiana <hugues.kambampiana@arm.com>