xz: use ALTERNATIVES instead of copying symlinks

xz: use ALTERNATIVES instead of copying symlinks

Installing the 'xz' package currently leads to file collisions:
Collected errors:
 * check_data_file_clashes: Package xz wants to install file /usr/bin/unxz
        But that file is already provided by package  * busybox
 * check_data_file_clashes: Package xz wants to install file /usr/bin/xz
        But that file is already provided by package  * busybox
 * opkg_install_cmd: Cannot install package debootstrap.

Fix that by switching to use ALTERNATIVES for all multicall commands
instead of copying the symlinks into the package.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>