We have scripts (written in python) that are run after a binary image is
created in the tools. These scripts are not really exportable, as they
are part of the tools and may include and use any bits of python code
they please. So we don't export them. This patch disables export
combinations that would not work because the post-binary hook is not
exported. A white-list is used for forward compatibility.
Before, `make_gcc_arm` would export a makefile with both linker flags in
the `$(LD)` make variable and the `$(LD_FLAGS)` make variable. This only
affected GCC based makefiles, as the `$(LD)` make variable would only
contain the linker for both ARMC5 and IAR. This patch enforces the
`$(LD)` behavior of ARMC5 and IAR on all of the compilers by truncating
`$(LD)` to just the command
For toolchains that do not implicitly run the C pre-processor on their
linker scripts the toolchain is expected to define a `preproc`
attribute. The Makefiles then pick up on this attribute and add another
rule for pre-processing the linker script automatically.