Move the mbed_set_mbed_target_linker_script function into its own file
and make it set the linkerscript on the target passed in, rather than
setting the linkerscript on the top level mbed-core target. We use
CMake's automatic forwarding of usage requirements to forward the linker
script to the top level target.

This commit also adds a condition to check whether the MBED_TARGET is a
CMake buildsystem target. If it is we will just link to it in the
idiomatic CMake way, if it's not then we use the 'old' mechanism of
setting the linkerscript for the mbed-core target. This check will be
useful when we refactor MBED_TARGETS to be CMake buildsystem targets as
it will allow us to do the refactoring incrementally.