Only trim serial number for hostname

Serial numbers previously were limited to 4 characters everywhere. This
changes the configuration so that they're only limited for hostnames
where there's some benefit of making them easier to type. Everywhere
else can use the whole serial number or decide what part of it they
want.

rootfs_overlay/etc/boardid.config

@@ -16,5 +16,5 @@
 -b uboot_env -u nerves_serial_number
 -b uboot_env -u serial_number
 
-# Last resort: use 4 digits of the RPi's unique ID as the serial number
--b rpi -n 4
+# Default: use the the RPi's unique ID as the serial number.
+-b rpi

rootfs_overlay/etc/erlinit.config

@@ -70,10 +70,10 @@
 # Erlang release search path
 -r /srv/erlang
 
-# Assign a hostname of the form "nerves-<serial_number>".
+# Assign a hostname of the form "nerves-<last 4 chars of the serial number>".
 # See /etc/boardid.config for locating the serial number.
 -d /usr/bin/boardid
--n nerves-%s
+-n nerves-%-.4s
 
 # If using shoehorn (https://github.com/nerves-project/shoehorn), start the
 # shoehorn OTP release up first. If shoehorn isn't around, erlinit fails back