Revert "core: also display the custom help with our main help"

Revert "core: also display the custom help with our main help"

This reverts commit 0a767deba004c920e7824543380cffbb298507f8.

Turns out that the custom help is not available when the $(O) directory
has not been configure yet (i.e. when there is no .config already
filled).

Rather than trying to work around this limitation with dirty hacks, just
revert this feature. After all, this will not prevent an external.mk
from providing custom help anyway; it's just not gonna be advertised nor
displayed with the main help.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Cc: Jérôme Pouiller <jezz@sysmic.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>