Squashed commit of the following:
arch/arm/src/tiva/Make.defs: Trivial change to conditional VPATH.
arch/arm/src/tiva: Move LM3S and LM4F source files to lm/ sub-directory. Move all TM4C source files to tm4c sub-directory.
arch/arm/src/tiva and configs/: Change include patch from chip/ to hardware/ in all Tiva files includes.
arch/tmp/src: Rename chip/ subdirectory to hardware/. This is a better name since it does not conflict with other directory names and, well, we are going to be change a lot of the Tiva directory structure in the next few commits.
