ar71xx: Fix installation of fw_setenv in sysupgrade ramdisk

ar71xx: Fix installation of fw_setenv in sysupgrade ramdisk

The install_bin from /lib/upgrade/common.sh is no longer creating the
symlinks when a secondary parameter is added. But the fw_setenv program was
always copied this way to the ramdisk for the upgrade.

Instead, just install fw_setenv and let install_bin handle the detection of
the required dependencies.

Fixes: 438dcbfe74a6 ("base-files: automatically handle paths and symlinks for RAMFS_COPY_BIN")
Signed-off-by: Sven Eckelmann <sven.eckelmann@openmesh.com>