cmake: add documentation about how it is built

cmake: add documentation about how it is built

Commit 7b17bafc5d7948aff3059e058ada80ad1fc50500 by Davide Viti has a
detailed explanation of some unusual techniques used for building
host-cmake and (target-)cmake. This is useful information for whoever
starts hacking on it, so copy it in the makefile, where it will be
easily noticed.

Also remove the sentence about host-cmake having a runtime dependency
on host-pkgconfig (not true anymore: it's the specific cmake-packages
that depend on it) and fix typos.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Cc: Samuel Martin <s.martin49@gmail.com>
Cc: Davide Viti <zinosat@tiscali.it>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>