getenv_string broken?

I have a getenv_string() that is never being evaluated.  I declared it as
follows:

static char acpi_os_name[128];
TUNABLE_STR("hw.acpi.os_name", acpi_os_name, sizeof(acpi_os_name));

The contents of acpi_os_name are never filled in from the variable I
declared in /boot/loader.conf. Just to be sure, I made a loop that prints
out the contents of kenvp.  It does not have hw.acpi.os_name.

Here is a snippet of the kenvp array, right where my tunable should be
(os2 is from a test below):

name: hint.vt.0.disabled=1
name: hw.acpi.os2=8
name: hw.acpi.reset_video=0
name: hw.acpi.verbose=1
name: hw.cbb.start_memory=0xc0203000

For another test, I declared a tunable int right below the string, in the
same source file.  It gets evaluated and assigned.

static int acpi_os2 = 2;
TUNABLE_INT("hw.acpi.os2", &acpi_os2);

-Nate