Move to toolchain file workflow

MCU-Driver-HAL has been modified to use a standard toolchain file
workflow. The requirement for including app.cmake from the HAL has been
removed. We now expect users to set CMAKE_TOOLCHAIN_FILE with the
toolchain file needed. The toolchain files have been moved out of the
HAL to a separate repo, which is added as a submodule to the HAL for
convenience.

Update MCU-Driver-HAL submodule so we can use the toolchain file
workflow.

Remove app.cmake as it no longer exists. Include the mbed_config.cmake
files directly rather than relying on app.cmake to include those config
files.

Update the CI scripts to use toolchain files instead of setting
MBED_TOOLCHAIN.

Signed-off-by: Robert Walton <rob.walton@arm.com>