Build: core/Makefile - Major overhaul

[HeckHeckHeckHeck]

Aims And Claims

• Build is fully customizable now from the "User Project" without any changes required in this repo
• Fully backwards compatible with prior version (regression tested with latest DaisyExamples and DaisyBlinkProject)
• Separate interface from implementation and present it with more detailed documentation on how to use

Changes

• Add Makefile.conf - declarative build settings interface
• Fix: support of user defined CFLAGS, CXXFLAGS, ASFLAGS, LDFLAGS
• Fix: support build config from shell env vars
• Fix: debug symbols now only in DEBUG build
• Feature: support user defined suffix for c++ files (USER_SUFFIX_CXX)
• Provide structure and process to remove items from flags and lists (not just appending to it)
• Structure the customization into three documented levels of build customization needs (Required, Optional, DangerZone)
• More error handling of build mis-configuration (warn/error messages)

New Build Options

• STARTUP_CODE {ASM, C}
• USER_SUFFIX_CXX
• C_STANDARD
• CXX_STANDARD
• GCC_SPECS
• new APP_TYPE 'CUSTOM' - allows custom linkerscript
• LDSCRIPT