Unlike COMPILE_OPTIONS, COMPILE_DEFINITIONS CMake property assumes
values without the -D prefix, such as NAME or NAME=VAL.
Previously, IDF build system was passing COMPILE_DEFINITIONS build
property to CMake COMPILE_OPTIONS property, so -D prefix was not
a problem.
Now that COMPILE_DEFINITIONS CMake property is used, -D prefix has
to be removed.
(Note that this doesn't affect 'target_compile_definitions' function,
which strips -D prefix before adding the definition to the property.)
