Makefile.freertos: verbatim inclusion of cpu/arm/stm32-ocm3/Makefile.stm32-ocm3

This is a starting point for future cleanup and simplification.
Makefile.freertos: remove MCK

The hybrid shouldn't have this kind of delay loops on the Contiki side.
Makefile.freertos: remove all references to libopencm3

Interestingly, things were clean enough that we didn't have any dependencies
on it, not even for things like register accesses.
Makefile.freertos (LINKERSCRIPT): remove, along and all references to it

Since we don't link the final executable we don't need a linker script
either.
Makefile.freertos: booby trap LDFLAGS; remove references to it

Since we don't produce the final executables, anything involving the linker
is suspicious. contiki-freertos.a is build by $(CC), not $(LD).
Makefile.freertos: remove anything referencing *.$(TARGET)

Again, these would be executables, the generation of which we've removed in
the previous step.
Makefile.freertos: remove tools, commented-out things, unnecessary sanity check
Makefile.freertos: remove ARM-side CONTIKI_CPU and its tributaries

We set CONTIKI_CPU twice, first for ARM, then for -freertos, leaving the
first setting without effect besides the possible creation of confusion.
Makefile.freertos: remove all references to stripping, and OBJCOPY as well
Makefile.freertos (CONTIKI_SOURCEFILES): we have it twice - remove one of them
Makefile.freertos: remove {elf,}loader, CUSTOM_RULE_LINK

No need to worry about starting out of an ELF file. No need to worry about
starting at all since FreeRTOS does it for us, for that matter.
Makefile.freertos: split CPU-specific items off into cpu/cm4.mk
Makefile.freertos, cpu/cm4.mk: remove CDEPFLAGS, STM32OCM3, clean up comments
Makefile.freertos, cpu/cm4.mk: move last dash to PREFIX

That way, an empty PREFIX will use the host's default toolchain.