findroot hook: use initcpio cmdline variable

Arch's initcpio automatic creates global variables in the form of
variable=y to kernel command line options without a suffix.

See parse_cmdline_item():

https://git.archlinux.org/mkinitcpio.git/tree/init_functions

airootfs/etc/initcpio/hooks/findroot

@@ -1,5 +1,5 @@
 run_hook() {
-    grep -q 'findroot' /proc/cmdline || return
+    [[ "${findroot}" == "y" ]] || return
 
     # Initialisation
     local newroot="/new_root"