Patch1:
Changed name of the variables.
Btw this is how it is all connected:

This enables Legato to build doxygen and include a link to a mainpage
from another project.
The other system must define the following

export EXTERNAL_DOXYGEN_DEFINE = EXTERNAL_DOXYGEN_DEFINE
export EXTERNAL_DOXYGEN_ROOT = $(PROJECTX_ROOT)/apps/WiFi
and it must also have a doxygen page called "external_proj_mainpage"


These defines are defined in targetDefs.projectx and will be read by CMake.

framework/doc/Cmakelist.txt:
EXTERNAL_DOXYGEN_DEFINE
EXTERNAL_DOXYGEN_ROOT
and sets
CMAKE_EXTERNAL_DOXYGEN_ROOT

They are then transfered into the soon to be preproccessed
doxygen config files :
framework/doc/doxygen.implementation.cfg.in
framework/doc/doxygen.user.cfg.in

These doxygen configuration files are preprocessed by CMake
and enables basic variable expansion.
Please note that doxygen variables are very restrictive
and only the ones set with the variable ENABLED_SECTIONS
in the config file are available.

This all enables the usage of conditional (similare to ifdef) usage to
include/exclude text. Currently all that is done is a link from the
Legato main page to the other projects page
Example:
@cond EXTERNAL_DOXYGEN_DEFINE
@subpage external_proj_mainpage <br>
@endcond

Change-Id: I8d9770dfef2d784a3221716e4a2a3fb725d9c90b