There's no need to guess at the COMPILER_TYPE to pass it down. We
guess wrong for buildkernel when CC=gcc49, say. Eliminate all the
guessing. COMPILER_TYPE propigates properly on its own, if specified,
and we guess it correctly otherwise lower in the build. Also, fix
conditionals for armv6hf when using an external compiler chain. They
were broken before, but unused. Also, prefer checking the compiler
type over CLANG_IS_CC since the latter is only supposed to be used to
determine what symlinks to install (more fixes to follow).
Remove instances of variables that were set, but never used. gcc 4.9
warns about these by default.