The tar command and its arguments are repeated for many archive types in
the unpack function. Unify the common parts in a variable to prepare
further extension.
(Bitbake rev: a08e57c9eaec1d9740a96149bf4843e576da4e5c)
Signed-off-by: Stefan Herbrechtsmeier <stefan.herbrechtsmeier@weidmueller.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>