Makefile: Unexport TAR_OPTIONS

GNU tar > 1.13.17 interprets TAR_OPTIONS as an environment variable
containing options to be prepended to the set on the command line.

Since we use the same variable, if the user's environment already
contains TAR_OPTIONS, our use of the same variable name modifies
the environment and causes untars to misbehave when TAR_OPTIONS
causes a -xf to be prepended to the tar command line, likely
converting a subsequent flag into a spurious filename.

Signed-off-by: Nick Alcock <nick.alcock@oracle.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>