unified-system-plugin: Change to include files in sensord package by flag

unified-system-plugin: Change to include files in sensord package by flag

Instead of separating unified-system-plugin files into a package, files
are included in the main package 'sensord' according to a flag named
enable_unified_system_service.

If flag is 1 then:
    libunified-system-sensor.so is included
    Unnecessary files for unified-system-plugin(sensord executable,
    service files, socket files) are excluded

If flag is not 1 then:
    libunified-system-sensor.so is excluded
    sensord executable, service files, socket files are included

Change-Id: I82ef689e00b85da81dbd39819dac4921921861e6
Signed-off-by: SangYoun Kwak <sy.kwak@samsung.com>