buildroot

fs/iso9660: rename internal variable

We're soon to introduce a generic variable with that name, i.e.
$(FS_NAME)_TARGET_DIR, so our internal, temporary variable would
clash with it.

Just rename it.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>