kbuild: rpm-pkg: introduce %{with_devel} switch to select devel package

scripts/package/mkspec preprocesses the spec file by sed, but it is
unreadable. This commit removes the last portion of the sed scripting.

Remove the $S$M prefixes from the conditionally generated lines.
Instead, surround the code with %if %{with_devel} ... %endif.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>