Add missing declaration of override_install_dir

archiso/mkarchiso:
In d90184a7 the unbound variable override_install_dir was introduced by accident.
It is required to be set to empty string to provide override functionality for install_dir.

Fixes #57

archiso/mkarchiso

@@ -33,6 +33,7 @@ iso_application="${app_name} iso"
 override_iso_application=""
 iso_version=""
 install_dir="${app_name}"
+override_install_dir=""
 arch="$(uname -m)"
 pacman_conf="/etc/pacman.conf"
 override_pacman_conf=""