multilib: avoid expanding grub and grub-efi to multilib

multilib: avoid expanding grub and grub-efi to multilib

It doesn't make much sense to expand them to multilib, and there is an error on
qemuarm64 since grub-efi supports arm64, but doesn't support armv7a or armv7ve:

* Fixed:
  MACHINE = "qemuarm64"
  require conf/multilib.conf
  MULTILIBS = "multilib:lib32"
  DEFAULTTUNE_virtclass-multilib-lib32 = "armv7a"
  MACHINE_FEATURES_append = " efi"

  $ bitbake lib32-core-image-minimal

Also introduced a variable NON_MULTILIB_RECIPES in multilib.conf, so that we
can easily add other recipes, such as syslinux if needed.

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>