kbuild: rpm-pkg: clean up mkspec

Clean up the mkspec without changing the behavior.

 - grep CONFIG_DRM=y more simply

 - move "EXCLUDE" out of the "%install" section because it can be
   computed when the spec file is generated

 - remove "BuildRoot:" field, which is now redundant

 - do not mkdir $RPM_BUILD_ROOT/lib/modules explicitly because it
   is automatically created by "make modules_install"

 - exclude "%package devel" from source package spec file because
   it does not make sense where "%files devel" is already excluded

 - exclude "%build" from source package spec file

 - remove unneeded "make clean" because we had already cleaned
   before making tar file

 - merge two %ifarch ia64 conditionals

 - replace KBUILD_IMAGE with direct use of $(make image_name)

 - remove trailing empty line from the spec file

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