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>

package/cmake/cmake.mk

@@ -10,6 +10,18 @@ CMAKE_SITE = https://cmake.org/files/v$(CMAKE_VERSION_MAJOR)
 CMAKE_LICENSE = BSD-3c
 CMAKE_LICENSE_FILES = Copyright.txt
 
+# CMake is a particular package:
+# * CMake can be built using the generic infrastructure or the cmake one.
+#   Since Buildroot has no requirement regarding the host system cmake
+#   program presence, it uses the generic infrastructure to build the
+#   host-cmake package, then the (target-)cmake package can be built
+#   using the cmake infrastructure;
+# * CMake bundles its dependencies within its sources. This is the
+#   reason why the host-cmake package has no dependencies:, whereas
+#   the (target-)cmake package has a lot of dependencies, using only
+#   the system-wide libraries instead of rebuilding and statically
+#   linking with the ones bundled into the CMake sources.
+
 CMAKE_DEPENDENCIES = zlib jsoncpp libcurl libarchive expat bzip2 xz
 
 CMAKE_CONF_OPTS = \