pkg-infra: add host-xxx-package macro

Create host-generic-package, host-autotools-package and
host-cmake-package macros.  Such a macro is more intuitive to use than
the $(call ...,host) construct.  Also it speeds things up by having
one less $(call ...) evaluation.

Also includes documentation update, but not for buildroot.html.

This brings the time for 'make -qp' (which is used by bash-completion)
down from 1.85s to 1.35s on my laptop.

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