toolchain: copy_toolchain_lib_root: clarify input parameter

The input to copy_toolchain_lib_root is not one library, not a list of
libraries, but a library name pattern with glob wildcards.
This pattern is then passed to 'find' to get the actual list of libraries
matching the pattern. Reflect this using an appropriate variable name.

Note: if the root of the buildroot tree contains a file matching one of
these library patterns, the copying of libraries from staging to target will
not be correct. It is not impossible to fix that, e.g. using 'set -f', but
maybe it's not worth it.

Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>