core: add the possibility to provide help for custom rules

When using a br2-external tree, it is possible (as stated in our manual)
to implement whatever arbitrary extra make rules (such as flashing a
board, or extracting the rootfs in an NFS export...). Some of those
extra rules might be exposed to the user as new entry points that the
user can call by itself.

However, there is no way for the br2-external to advertise those new
rules in the help text.

We add the possibility to do so, by adding a new make rule, called
help-custom, advertised in our own help info.

It is up to the br2-external tree to provide whatever help text is
deemed necessary. The format of the help is completely free-form.

Note that we need to provide an empty, dummy help-custom rule, since it
is always advertised (making it .PHONY does not work). Since this rule
is empty, make gently reports that there is "Nothing to be done for
`help-local'", which is pretty well fitting when help-local was not
provided (either because there's no br2-external tree, or when the
br2-external tree does not provide it.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Jérôme Pouiller <jezz@sysmic.org>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

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