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.

Fixes: 3989a3d81701 ("loader: Only warn when we have neither FDT nor ACPI")
Sponsored by: Amazon