allow /dev/sd??/nvram nvram for arm machines.
perhaps we should have overall defaults.

Reference: /n/atom/patch/applied2013/armsdnvram
Date: Thu Nov 14 20:29:55 CET 2013
Signed-off-by: quanstro@quanstro.net

--- /sys/src/libauthsrv/readnvram.c	Thu Nov 14 20:29:17 2013
+++ /sys/src/libauthsrv/readnvram.c	Thu Nov 14 20:29:17 2013
@@ -38,6 +38,7 @@
 	"power", "/nvram", 0, sizeof(Nvrsafe),	/* OK for Ucu */
 	"arm", "#F/flash/flash0", 0x100000, sizeof(Nvrsafe),
 	"arm", "#F/flash/flash", 0x100000, sizeof(Nvrsafe),
+	"arm", "#S/%s%x/nvram",	0, 512,
 	"debug", "/tmp/nvram", 0, sizeof(Nvrsafe),
 };