Only trim serial number for hostname

Serial numbers previously were limited to 8 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

@@ -1,8 +1,8 @@
 # boardid.config
 
-# Read the D1's SID and use the last 8 digits (turns out 4 isn't sufficient)
+# Read the D1's SID
 # NOTE: I couldn't find good information on how reliable this method was in
 #       producing a unique ID. The SID info in the D1's datasheet was on
 #       accessing the SID and not on how the eFuse's were programmed. Please
 #       let me know if you find definitive info on this.
--b binfile -f /sys/bus/nvmem/devices/sunxi-sid0/nvmem -l 16 -k 0 -n 8
+-b binfile -f /sys/bus/nvmem/devices/sunxi-sid0/nvmem -l 16 -k 0

rootfs_overlay/etc/erlinit.config

@@ -1,4 +1,8 @@
 # Additional configuration for erlinit
+#
+# To override the settings in this file, see
+# https://hexdocs.pm/nerves/advanced-configuration.html#overriding-erlinit-config-from-mix-config.
+#
 
 # Turn on the debug prints
 #-v
@@ -61,10 +65,10 @@
 # Erlang release search path
 -r /srv/erlang
 
-# Assign a hostname of the form "nerves-<serial_number>".
+# Assign a hostname of the form "nerves-<last 8 chars of the serial number>".
 # See /etc/boardid.config for locating the serial number.
 -d /usr/bin/boardid
--n nerves-%s
+-n nerves-%-.8s
 
 # If using shoehorn (https://github.com/nerves-project/shoehorn), start the
 # shoehorn OTP release up first. If shoehorn isn't around, erlinit fails back