Set module_var variable as a CACHE variable

:Release Notes:
The MODULE_* variables should be CACHE variables for their values to be used
within sub-directories.

:Detailed Notes:
When the MODULE_* variables were being set as normal variables, their
values where not being used within the sub-directories (rockhopper/generic)
so none of the modules in NYX_MODULES_REQUIRED string were being built.
With this change now all the required modules are being built. Since the
"FORCE" option is being used, the values cannot be overridden by any other
meta-layer.

:Testing Performed:
Verified that the specified modules are now being built.

:Issues Addressed:
[CTO-188] Modify the CMake Script

Open-webOS-DCO-1.0-Signed-off-by: Sapna Todwal <sapna.todwal@lge.com>

Change-Id: Ie59632b608cfc1f5dfe42757ccc8dad804de12a0
Reviewed-on: https://g2g.palm.com/6281
Reviewed-by: DCO Verification
Tested-by: Sapna Todwal <sapna.todwal@lge.com>
Reviewed-by: Sapna Todwal <sapna.todwal@lge.com>
Merge remote-tracking branch 'origin/develop'