hal-backend-service: Add EnvironmentFile=/run/xdg-root-env to use libtbm

hal-backend-service: Add EnvironmentFile=/run/xdg-root-env to use libtbm

Add EnvironmentFile=/run/xdg-root-envto hal-backend-service process
because  hal-backend-service plugin might want to use libtbm (Tizen Buffer Manager)
for handling buffer such as graphics, camera, codec buffers.

Require the XDG_RUNTIME_DIR environment variable if hal-backend-service
plugin uses libtbm library. But, XDG_RUNTIME_DIR environment variable
is not added to /run/tizen-system-env or global exported environment
data. Instead, enlightenment exports XDG_RUNTIME_DIR environment variable
to /run/xdg-root-env file.

Change-Id: I1ebf726ae1bf3e3d9a0bbe83cf28f3b5efb798e4
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>