sandbox: Add a build for VPL

Add an initial VPL build for sandbox. This includes the flow:

   TPL (with of-platdata) -> VPL -> SPL -> U-Boot

To run it:

   ./tpl/u-boot-tpl -D

The -D is needed to get the default device tree, which includes the serial
console info.

Add a Makefile check for OF_HOSTFILE which is the option that enables
devicetree control on sandbox.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/cpu/spl.c b/arch/sandbox/cpu/spl.c
index 3f107b8..fe5d44d 100644
--- a/arch/sandbox/cpu/spl.c
+++ b/arch/sandbox/cpu/spl.c
@@ -32,13 +32,21 @@
 	return 0;
 }
 
-/* SPL / TPL init function */
+/* SPL / TPL / VPL init function */
 void board_init_f(ulong flag)
 {
 	struct sandbox_state *state = state_get_current();
+	int ret;
 
 	gd->arch.ram_buf = state->ram_buf;
 	gd->ram_size = state->ram_size;
+
+	ret = spl_early_init();
+	if (ret) {
+		debug("spl_early_init() failed: %d\n", ret);
+		hang();
+	}
+	preloader_console_init();
 }
 
 u32 spl_boot_device(void)
@@ -75,8 +83,6 @@
 {
 	struct sandbox_state *state = state_get_current();
 
-	preloader_console_init();
-
 	if (state->run_unittests) {
 		struct unit_test *tests = UNIT_TEST_ALL_START();
 		const int count = UNIT_TEST_ALL_COUNT();