kbuild: rpm-pkg: refactor mkspec with here doc

The repeat of echo is unreadable.  The here-document is a well-known
device for such scripts.  One difficulty is we have a bunch of PREBUILT
conditionals that would split the here-document.

My idea is to add "$S" annotatation to lines only for the source package
spec file, then post-process it by sed.  I hope it will make our life
easier than repeat of "cat <<EOF ..."

I confirmed this commit still produced the same (bin)kernel.spec as
before.

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