loader: use CMAKE_INSTALL_FULL_<dir> to improve Linux filesystem compatibility

loader: use CMAKE_INSTALL_FULL_<dir> to improve Linux filesystem compatibility

In order to improve the loader's compatibility with different filesystem hierarchies
under Linux, use CMake's CMAKE_INSTALL_FULL_<dir> instead of CMAKE_INSTALL_<dir> to
correctly resolve the absolute paths of SYSCONFDIR and DATADIR in compliance with
the GNU Coding Standard [1] and Linux Foundation's Filesystem Hierarchy Standard [2],
as explained in CMake's documentation [3].

Note to packagers: When using CMAKE_INSTALL_PREFIX, this commit obviates the
definition of other CMAKE_INSTALL_ defines to properly detect ICD and layer
manifests in GNU/Linux-compliant filesystems.

This commit fixes GH-1083. (see the issue for more details)

[1] https://www.gnu.org/prep/standards/html_node/Directory-Variables.html
[2] https://refspecs.linuxfoundation.org/FHS_3.0/fhs/index.html
[3] https://cmake.org/cmake/help/v3.4/module/GNUInstallDirs.html

Change-Id: If743a3c8f5f7381e00c0d23b95b9e13c0dead144
Signed-off-by: Dor Askayo <dor.askayo@gmail.com>