package/qemu: unset TARGET_DIR

qemu uses TARGET_DIR internally, and it is at least used to display the
shortened compiling commands, like (with a TARGET_DIR=/path/to/target):

    CC /path/to/targetblock/write-threshold.o

VS

    CC block/write-threshold.o

There does not seem to be any adverse effect to that, but this is very
confusing to see, especially when building the host variant.

Fix that by unsetting TARGET_DIR prior to building.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Francois Perrad <francois.perrad@gadz.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>