build/nano: brave early tester said too big for some SD cards

Let's tone this down considerably, we have growfs to fix this
afterwards anyway.

README.md

@@ -214,8 +214,7 @@ A verification of all available set signatures is done via:
 
     # make verify
 
-Nano images can be adjusted in size using an argument as follows,
-with the default of 3800M:
+Nano images can be adjusted in size using an argument as follows:
 
     # make nano-<size>
 

build/nano.sh

@@ -34,7 +34,7 @@ SELF=nano
 
 check_images ${SELF} ${@}
 
-NANOSIZE="3800M"
+NANOSIZE="3500M"
 
 if [ -n "${1}" ]; then
 	NANOSIZE=${1}