ArmPkg/Library: prevent endless reboot loop with emulated NV varstore

ArmPkg/Library: prevent endless reboot loop with emulated NV varstore

If no valid boot options were found, PlatformBootManagerLib refreshes a
set of sane default options and then reboots. However, if there is in
fact no persistent varstore, the same thing happens again on next boot,
and we end up in an endlessly rebooting loop.

So when PcdEmuVariableNvModeEnable is TRUE, skip the reboot step and
enter the setup menu instead.

Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Leif Lindholm <leif@nuviainc.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>