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>

# Options specific to crosstool-NG acting as a backend

config IS_A_BACKEND

    string

    option env="CT_IS_A_BACKEND"

config BACKEND

    bool

    default y if IS_A_BACKEND = "y" || IS_A_BACKEND = "Y"

config BACKEND_ARCH

    string

    option env="CT_BACKEND_ARCH"

config BACKEND_KERNEL

    string

    option env="CT_BACKEND_KERNEL"

config BACKEND_LIBC

    string

    option env="CT_BACKEND_LIBC"