kernel-devsrc: do not copy Module.symvers file during install

kernel-devsrc: do not copy Module.symvers file during install

When CONFIG_MODULES is not enabled in kernel config - Module.symvers
generation is not done, which causes the file not to be created.

This fails later in do_install() due to the fact that copy
command in executed for non-existing Module.symvers file.

Check for Module.symvers existence before copy command in executed.

Signed-off-by: Oleksandr Ocheretnyi <oocheret@cisco.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>