Makefile: help: add package-specific targets

The documentation of the package-specific targets is heavily inspired by
the earlier patch by Thomas Petazzoni [1], but the <pkg>-install*
targets were left out since they're not useful for the developer IMHO.

The *-menuconfig target's help text is moved to the package-specific
targets.

[1] http://patchwork.ozlabs.org/patch/433692



[Thomas: remove double <pkg>-graph-depends help.]

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>