configs/releng: move the mirror uncommenting sed command from...

configs/releng: move the mirror uncommenting sed command from customize_airootfs.sh to a pacman hook

After pacman-mirrorlist is installed, /etc/pacman.d/hooks/uncomment-mirrors.hook will run a sed command which uncomments all Server lines in /etc/pacman.d/mirrorlist.
This brings us another step closer to the complete removal of customize_airootfs.sh.

Related to #21 .

configs/releng/airootfs/etc/pacman.d/hooks/uncomment-mirrors.hook

+[Trigger]
+Operation = Install
+Operation = Upgrade
+Type = Package
+Target = pacman-mirrorlist
+
+[Action]
+Description = Uncommenting all mirrors in /etc/pacman.d/mirrorlist...
+When = PostTransaction
+Depends = pacman-mirrorlist
+Depends = sed
+Exec = /usr/bin/sed -i "s/#Server/Server/g" /etc/pacman.d/mirrorlist

configs/releng/airootfs/root/customize_airootfs.sh

@@ -8,5 +8,3 @@ set -e -u
 
 sed -i 's/#\(en_US\.UTF-8\)/\1/' /etc/locale.gen
 locale-gen
-
-sed -i "s/#Server/Server/g" /etc/pacman.d/mirrorlist