Add PATH symbols to shared-macros.mk
The PATH variable is set in many of the component Makefiles. This change proposes defining two PATH symbols in shared-macros.mk that can be used in any of the component Makefiles. Once the component Makefiles are changed, it becomes possible to change PATH values in shared-macros.mk without any further change to component Makefiles. The default PATH itself is unchanged, making this a transparent change.