wic: allow imager-specific filename extensions

wic: allow imager-specific filename extensions

Currently there is only one defined imager as part of oe-core's wic
implementation: 'direct'. However, having a highly plugin-based design,
wic allows users to define their own imagers (and sources).

Therefore don't hard-code the filename extension of the imager output to
be 'direct' (i.e. the default imager). Allow the extension to follow the
name of the imager being used.

A user can specify a custom imager via the WIC_CREATE_EXTRA_ARGS variable.
If the user does not specify an imager, then 'direct' is assumed.

Signed-off-by: Trevor Woerner <twoerner@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>