Make it possible to use the env kernel config file option for AVILA

and CAMBRIA boards that does not use loader to load the kernel. This
is basically how it was done for i386. This way tunables can also be
set. For example in config file:

env "/conf/AVILA.env"

And in AVILA.env:

vfs.unmapped_buf_allowed=0

MFC after:	2 weeks

sys/arm/xscale/ixp425/avila_machdep.c

@@ -227,6 +227,8 @@ initarm(struct arm_boot_params *abp)
 	pcpu_init(pcpup, 0, sizeof(struct pcpu));
 	PCPU_SET(curthread, &thread0);
 
+	if (envmode == 1)
+		kern_envp = static_env;
 	/* Do basic tuning, hz etc */
       	init_param1();