Allow makefile to be used directly for cross-compiling (e.g. when treated as a git submodule). This requires CC and AR to be passed for a particular toolchain, but CFLAGS must also specify CPU and FPU options -> introduce MODE variable. CFLAGS cannot be passed without need to override in the makefile, so a new variable is needed.

Cortex-M7 Example (linker: library and project use of VFP registers must match):
CC=arm-none-eabi-gcc AR=arm-none-eabi-ar MODE="-mcpu=cortex-m7 -mthumb -mfpu=fpv5-sp-d16 -mfloat-abi=hard"

(commit can be cherry-picked)