force to use local-rootfs option
force to use local-rootfs option

We do not provide downloading the rootfs tarball from github release
tag anymore. Instead of it, the tarball should be downloaded from
artik.io and should be specified through --local-rootfs option.

Change-Id: Ibc39e56d31c9d0969a0008ac23e97be78c240684
Signed-off-by: Chanho Park <8ec285a53e21b773fd914e03c50e1b6431bc9aa8@samsung.com>

SamsungARTIK/build-artik