kbuild: remove 'make /' support

'make /' is just an alias for 'make ./'; this builds all objects of an
external module, but skips the modpost stage.

I am not a big fan of 'make /' since it looks as if it were touching
the root directory of the system. I like 'make ./' better.

I do not know how many people are using it, but let's show a hint if
it is used. Also, move it close to the external module rules since
this only makes sense for external modules.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>