toolchain: copy_toolchain_lib_root: clarify logic

toolchain: copy_toolchain_lib_root: clarify logic

copy_toolchain_lib_root has slightly different logic depending on the type
of library object: file or link. All actions related to links are not
relevant in case you are working with a file. Hence, try to increase clarity
by not executing unnecessary lines in the 'file' case.

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