fix spurious / on toplevel kernel module installations

When kernel modules are being put directly into lib/modules/ (as opposed
to lib/modules/<subdir>/), the empty part of the destination path causes
a target containing a redundant /. If the target is also added elsewhere
in the build without the redundant /, then rule merging will miss it,
and a build error will occur.

Bug: 158158635
Change-Id: I797addce760f4886166efa283302a53783581625