loader: Only warn for !FDT && !ACPI take two

For unclear reasons, we used to print two warnings about not having a
DTB: First,
	printf("No valid device tree blob found!\n");
and immediately later a second,
	printf("WARNING! Trying to fire up the kernel, but no "
	    "device tree blob found!\n");

A previous commit silenced the second warning in the event that the
acpi.revision environment variable was set, on the theory that systems
which have ACPI might not need a DTB; this commit silences the first
of the two warnings under the same condition.

Reviewed by:	andrew
Fixes:	3989a3d817 ("loader: Only warn when we have neither FDT nor ACPI")
Sponsored by:	Amazon
Differential Revision:	https://reviews.freebsd.org/D54772

stand/fdt/fdt_loader_cmd.c

@@ -993,7 +993,7 @@ fdt_copy(vm_offset_t va)
 	debugf("fdt_copy va 0x%08jx\n", (uintmax_t)va);
 	if (fdtp == NULL) {
 		err = fdt_setup_fdtp();
-		if (err) {
+		if (err && (getenv("acpi.revision") == NULL)) {
 			printf("No valid device tree blob found!\n");
 			return (0);
 		}