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This caused to gain the following error when building:

Only one of PREFIX or INSTALL_BASE can be given. Not both.

Using INSTALLDIRS=vendor is the common way to get the modules installed
into the right directories.

Fixes #12573.
Signed-off-by: Stefan Schantl <stefan.schantl@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>