hv: remove CONFIG_PARTITION_MODE for pre-launched VM vE820 creation

Preparing for hybrid mode:

- create vE820 for pre-launched VMs and do other init code when the
  vm_config->type is PRE_LAUNCHED_VM.
- create ve820.c for each board because without wrapping by
  CONFIG_PARTITION_MODE, ve820_entry[] needs to be visible even when
  compiling target boards that haven't enabled pre-launched VMs.
- remove create_prelaunched_vm_e820() from vm.c and implement board
  specific function for each $(CONFIG_BOARD)/ve820.c. The reasons being:
  - don't need to define ve820_entry[32] for those boards that don't
    support pre-launched VMs.
  - more importantly, this makes it much easier to create different per-VM
    vE820 when it's needed.

Tracked-On: #2291
Signed-off-by: Zide Chen <zide.chen@intel.com>

hypervisor/arch/x86/configs/nuc6cayh/ve820.c

@@ -0,0 +1,16 @@
+/*
+ * Copyright (C) 2019 Intel Corporation. All rights reserved.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+#include <vm.h>
+
+/**
+ * @pre vm != NULL
+ */
+void create_prelaunched_vm_e820(struct acrn_vm *vm)
+{
+	vm->e820_entry_num = 0U;
+	vm->e820_entries = NULL;
+}