comgt-ncm: Add possibility to choose PDP context type

comgt-ncm: Add possibility to choose PDP context type

By setting the option pdptype to IP, IPV6 or IPV4V6 the user can
choose the context type between IPv4, IPv6 and dual stack,
respectively. The default setting is dual stack, except if option
ipv6=0 is specified, in which case IPv4 context is the default.
This allows for an out-of-the-box IPv6 support with modems
utilizing NCM-like protocols.

While we are at it, also add commands for Sierra DirectIP modems
(currently untested), which will allow us to drop the separate
comgt-directip package (once tested and verified working).

Signed-off-by: Matti Laakso <malaakso@elisanet.fi>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46844 3c298f89-4303-0410-b956-a3cf2f4a3e73