Fix fw_env.config inconsistency

This fixes an error when running fw_printenv that would break
Nerves.Runtime.KV.

Author: fhunleth

rootfs_overlay/etc/fw_env.config

@@ -1,11 +1,6 @@
 # fw_printenv and fw_setenv configuration
 #
-# This is only used for storing firmware metadata on this platform. Even though
-# the Beaglebone uses U-Boot, U-Boot is configured to use the uenv.txt file as
-# it does upstream at beagleboard.org.
-#
 # See fwup.conf for offset and size
 
 # Device name	Device offset	Env. size	Flash sector size	Number of sectors
-/dev/mmcblk0	0x100000	0x20000		0x200			16
-
+/dev/mmcblk0	0x100000	0x20000		0x200			256