Use valid label which is available in all targets

Use valid label which is available in all targets

Because RPi3 and ARTIK target don't set partition name, it is invalid
in RPi3's config file. Change it to filesystem label which is
available in RPi3 and ARTIK target.
In addition, all reference targets are using partition/filesystem
name in lowercase. So use it as a default.

Change-Id: I49da6831b8663118af698f4e0733a8cba8d972b8
Signed-off-by: Sunmin Lee <sunm.lee@samsung.com>