Merge changes I801cea04,I4abb6c9d,I3c1cc0ec,I1b6f69ad,Ic4086a1f into integration

* changes:
  refactor(build): pass TF_CFLAGS to the assembler
  refactor(build): absorb CFLAGS into TF_CFLAGS
  refactor(build): use a standard rule to run the preprocessor
  refactor(build): place all cflags setting in one place
  refactor(build): simplify ENABLE_LTO checking