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>

 # powerpc specific configuration file

 ## select ARCH_SUPPORTS_32

 ## select ARCH_SUPPORTS_64 if EXPERIMENTAL

 ## select ARCH_DEFAULT_32

 ## select ARCH_USE_MMU

 ## select ARCH_SUPPORT_ABI

 ## select ARCH_SUPPORT_CPU

 ## select ARCH_SUPPORT_TUNE

 ##

 ## help The PowerPC architecture, as defined by:

 ## help     http://www.ibm.com/developerworks/eserver/articles/archguide.html