grub-efi.bbclass: take into consideration of multilib

grub-efi.bbclass: take into consideration of multilib

When enabling multilib and building lib32-IMAGE which
uses grub-efi, the build fails with the following error.

  install: cannot stat '/PROJ_DIR/build/tmp-glibc/deploy/images/intel-x86-64/grub-efi-bootia32.efi': No such file or directory

The grub-efi is in NON_MULTILIB_SCRIPTS. That means we
will use 64bit grub-efi for lib32-IMAGE.

So take into consideration of multilib to fix this problem.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>