kbuild: export_report: read modules.order instead of .tmp_versions/*.mod

Towards the goal of removing MODVERDIR aka .tmp_versions, read out
modules.order to get the list of modules to be processed. This is
simpler than parsing *.mod files in .tmp_versions.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>