Pass CMAKE_BUILD_TYPE to external projects

For projects built via ExternalProject_Add, CMake does not pass on the
CMAKE_BUILD_TYPE variable. This means we have to, for example, set
TFM_CMAKE_ARGS to -DCMAKE_BUILD_TYPE=Debug to get debug builds of TF-M.
Now, we can just set CMAKE_BUILD_TYPE at the top level and external
projects will be built with the same build type.

Signed-off-by: Jaeden Amero <jaeden.amero@arm.com>