kbuild: rpm-pkg: fix build error when CONFIG_MODULES is disabled

When CONFIG_MODULES is disabled, make rpm-pkg / binrpm-pkg fails
with the following message:

  The present kernel configuration has modules disabled.
  Type 'make config' and enable loadable module support.
  Then build a kernel with module support enabled.

Do not install modules in the case.  Also, omit the devel package.

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