The linux filesystem is case sensitive, this was causing our nightly build to
fail when attempting to find the script with its lower case name. The
name of the file has been kept the same as this seems to be STMs
convention.

Follow the naming for other CMake targets, using prefix mbed-, lower case with dashes

Refactor all ST targets to be CMake buildsystem targets. This removes
the need for checking MBED_TARGET_LABELS repeatedly and allows us to be
more flexible in the way we include MBED_TARGET source in the build.

A side effect of this is it will allow us to support custom targets
without breaking the build for 'standard' targets, as we use CMake's
standard mechanism for adding build rules to the build system, rather
than implementing our own layer of logic to exclude files not needed for
the target being built. Using this approach, if an MBED_TARGET is not
linked to using `target_link_libraries` its source files will not be
added to the build. This means custom target source can be added to the
user's application CMakeLists.txt without polluting the build system
when trying to compile for a standard MBED_TARGET.

No impact, no change