DRYify makefile generators, Restructure build directory, Allow additional targets from projcet.yaml

[theotherjimmy]

The makefile classes (ARMCC, and GCC_ARM) both use a common superclass and share most code.
This should reduce matenince costs.

The build directory is now structured to exactly match the source directory.
This follows the principal of least surprise and allows the programmer of additional rules to elide some of the details as to how to locate sources.

Additional Targets (rules, actually) are now supported in all makefile-based templates.
For example to add the rule

$(OBJ_FOLDER)source/daplink/daplink.ld : source/daplink/daplink.lds | create_outputdir
    arm-none-eabi-cpp $^ -o $@ $(COMMON_FLAGS) $(INC_DIRS_F)"

to pre-process the linker script with the c pre-processor, one would have a configuration file like so:

tool_specific:
    make_gcc_arm:
        mcu:
            - cortex-m0
        linker_file: "$(OBJ_FOLDER)source/daplink/daplink.ld"
        misc:
            additional_targets:
                - "$(OBJ_FOLDER)source/daplink/daplink.ld : source/daplink/daplink.lds | create_outputdir \n\tarm-none-eabi-cpp $^ -o $@ $(COMMON_FLAGS) $(INC_DIRS_F)"

[theotherjimmy]

Should resolve #371

[theotherjimmy]

I will write up and add to the wiki how to add additional targets, what advantages they have over pre/post build scripts, and the gotchas that exist once this is merged.

[theotherjimmy]

Breaking into two PRs: one as a refactor, and the other as the Restructuring and additional_targets.