cmake: remove the use of PROPERTY_LINKER_SCRIPT_DEFINES

The support of PROPERTY_LINKER_SCRIPT_DEFINES has been broken since
the transition to CMake in 12f8f761.

The intention was probably to allow users / projects to adjust
PROPERTY_LINKER_SCRIPT_DEFINES by setting a CMake cache variable.

The implementation tests for the CMake variable (local scope or cache)
PROPERTY_LINKER_SCRIPT_DEFINES, but it never uses such CMake variable.

Instead it uses a CMake global property named
PROPERTY_LINKER_SCRIPT_DEFINES. CMake variables and CMake global
properties are two very different things, and therefore the current
implementation has never worked. The fact that no one has never noticed
this flaw, means that the feature has never been used as intended.

Simplify the code by removing the use of the global CMake property and
instead set the value of the property on the linker script
pre-processing invocation.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>