i386: Don't include lib/i386/reset.c in EFI builds

i386: Don't include lib/i386/reset.c in EFI builds

Commit 0ba90a7f0178 ("efi: Move grub_reboot() into kernel") broke
the build on i386-efi - genmoddep.awk bails out with message
  grub_reboot in reboot is duplicated in kernel
This is because both lib/i386/reset.c and kern/efi/efi.c now provide
this function.

Rather than explicitly list each i386 platform variant in
Makefile.core.def, include the contents of lib/i386/reset.c only when
GRUB_MACHINE_EFI is not set.

Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>