Proper support for DESTDIR and INSTALL_MOD_PATH

The environment variables DESTDIR and INSTALL_MOD_PATH must
be mutually exclusive.

https://www.gnu.org/prep/standards/html_node/DESTDIR.html
https://www.kernel.org/doc/Documentation/kbuild/modules.txt

This issue was discussed in this Buildroot thread:
https://lists.buildroot.org/pipermail/buildroot/2021-August/621350.html

I saw this behavior in other different projects, as:

• Yocto Project: https://www.yoctoproject.org/pipermail/meta-freescale/2013-August/004307.html

• Google IA Coral: https://coral.googlesource.com/linux-imx-debian/+/refs/heads/master/debian/rules

For the above reasons, INSTALL_MOD_PATH will be set as DESTDIR
by default.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: José Luis Salvador Rufo <salvador.joseluis@gmail.com>
Signed-off-by: Romain Naour <romain.naour@gmail.com>
Closes #12577