MIPS: fix *-pkg builds for loongson2ef platform

Since commit 805b2e1d ("kbuild: include Makefile.compiler only when
compiler is needed"), package builds for the loongson2f platform fail.

  $ make ARCH=mips CROSS_COMPILE=mips64-linux- lemote2f_defconfig bindeb-pkg
    [ snip ]
  sh ./scripts/package/builddeb
  arch/mips/loongson2ef//Platform:36: *** only binutils >= 2.20.2 have needed option -mfix-loongson2f-nop.  Stop.
  cp: cannot stat '': No such file or directory
  make[5]: *** [scripts/Makefile.package:87: intdeb-pkg] Error 1
  make[4]: *** [Makefile:1558: intdeb-pkg] Error 2
  make[3]: *** [debian/rules:13: binary-arch] Error 2
  dpkg-buildpackage: error: debian/rules binary subprocess returned exit status 2
  make[2]: *** [scripts/Makefile.package:83: bindeb-pkg] Error 2
  make[1]: *** [Makefile:1558: bindeb-pkg] Error 2
  make: *** [Makefile:350: __build_one_by_one] Error 2

The reason is because "make image_name" fails.

  $ make ARCH=mips CROSS_COMPILE=mips64-linux- image_name
  arch/mips/loongson2ef//Platform:36: *** only binutils >= 2.20.2 have needed option -mfix-loongson2f-nop.  Stop.

In general, adding $(error ...) in the parse stage is troublesome,
and it is pointless to check toolchains even if we are not building
anything. Do not include Kbuild.platform in such cases.

Fixes: 805b2e1d ("kbuild: include Makefile.compiler only when compiler is needed")
Reported-by: Jason Self <jason@bluehome.net>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>