uqmi: Add support for QMI-based mobile broadband modems

Many of the 4G/LTE and 3G modems utilize the QMI-protocol to control the
modem. At the moment there is no support for them in OpenWrt. This
patch adds support for them in the form of a netifd script and a
control utility. Tested with Huawei E398 and ZTE MF820D (which requires
a delay of ~30 s before responding to QMI commands). I put myself up as
the maintainer, feel free to change this if you desire.

Signed-off-by: Matti Laakso <malaakso@elisanet.fi>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@40868 3c298f89-4303-0410-b956-a3cf2f4a3e73