Fix build when TAR_OPTIONS env variable is set

The build system sets a make variable TAR_OPTIONS to the unpacking
command, i.e. "-xf -". Now if an environment variable with the same
name is set, the make variable is automatically exported to the
environment. The make variable is added to the tar command in the
makefile, and tar adds the environment variable. This results in a
command like "tar -c /some/dir -xf - -xf -" which of course doesn't
work. It is also difficult to spot as the second "-xf -" is not
visible on the command line.
I suggest this is fixed by unexporting TAR_OPTIONS as I see no use
of the evironment variable, and it is changed from the original
value anyway.

Signed-off-by: Jan Kardell <jan.kardell@telliq.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@42794 3c298f89-4303-0410-b956-a3cf2f4a3e73