So far we have been selecting which packages are compiled using clang
this patch changes the logic to use clang by default unless selecting

TOOLCHAIN = "gcc"

explicitly in the recipe

We have not yet enabled clang for native recipes, its only enabled for
target recipes needing cross compilation as of now.

Get rid of configuration file and move the code to clang.bbclass,
simplifies the logic

If needed to select gcc to be default system compiler set

TOOLCHAIN = "gcc" in local.conf

Signed-off-by: Khem Raj <raj.khem@gmail.com>