dependencies: ensure that DESTDIR isn't set when running Buildroot

Having DESTDIR set in the environment before running Buildroot creates
some funky problems in the build process. Prevent users from running
into this kind of troubles.

Cc: Michael Opdenacker <michael.opdenacker@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>