Add mcu-driver-hal after SDK project is declared

We previously added the MCU Driver HAL as the top level project. This
was necessary because MCU Driver HAL hard-coded the inclusion of its
toolchain files. Toolchain files need to be processed before any
`project` calls are made so CMake's built in compiler checks pass. MCU
Driver HAL's toolchain support has now been reworked; toolchain files
can now be passed in on the command line. We no longer need to add the
MCU Driver HAL before our SDK project is declared.

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