package/linux-tools: fixes build of iio with 4.14+ kernels

package/linux-tools: fixes build of iio with 4.14+ kernels

Since Linux kernel commit [1], the build of the iio tool has been
changed to use the common Linux tools build system. The installation
directory is now given by DESTDIR, like for all other Linux tools.

We keep the INSTALL_DIR environment in the 'install' target to be
compatible with kernels older than 4.14.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=18956cf2d78a8d4a5959e20240f04ce8d5a6c121

Signed-off-by: Julien BOIBESSOT <julien.boibessot@armadeus.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>