Makefile: make it work for auto-completion

The latest autocompletion calls 'make -qp' to get all the possible
targets. This currently fails, as our makefile is calling itself
again and again ad-libitum.

Fix it by detecting that the recursion level is not 0, and bail out
if so. It works so well that it has the side effect of showing only
the 'public' target, and hide our internal ones! :-)

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 # Bare metal config options

 ## select BARE_METAL

 ##

 ## help 'Bare metal' refers to those programs that run without any kernel.

 ## help

 ## help You probably want to say 'y' here if you plan to use your compiler

 ## help to build bootloaders. It is not yet suitable to build Linux kernels,

 ## help though, because the APCI stuff relies on the target C library headers

 ## help being available?!?!...