Refactor project variables

This branch refactors the variables section of project.conf such that:

• C/C++ flags can be specified on a per element basis (separately)
• C/C++ flags can also be specified on a per element basis for the definition of the "build" flags (also separately, C++ vs C flags)
• Base common C/C++ flags which can be specified for both C and C++ compilers are still declared as 'flags' variables, to avoid duplication of these declarations
  • This means the few places where you still see variables named %{*_flags}, these are flags which are used in both cflags and cxxflags final declarations
• The final set of build and target C/C++ flags are built out entirely in the project.conf variables section
  • This removes the need for verbose arch conditional statements in various files
  • Now instead, one can just refer to build_cflags and target_cflags and know that they contain any architecture specific flags already
  • As such, conditionals have been removed from the project.conf environment section.
• Similar to compiler flags, there is now a local_ldflags which can be specified on a per element basis

This is a follow up from the conversation which took place in !1421 (merged)