add example for file on VFAT filesystem usage

For example on a raspberry pi the u-boot environment can be
saved in a file on the first VFAT partition.
This example illustrates how to use it with fw_printenv/fw_setenv.

Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>

tools/env/fw_env.config

@@ -20,3 +20,6 @@
 
 # Block device example
 #/dev/mmcblk0		0xc0000		0x20000
+
+# VFAT example
+#/boot/uboot.env	0x0000          0x4000