Use $((EXPRESSION)) instead of $[EXPRESSION]

$((EXPRESSION)) is POSIX-conform and should be preferred.

Change-Id: Ibd0524194aeda40961e202b6761dbaed2ea27dfa

lib/common-functions

@@ -20,7 +20,7 @@ function tmpfs_check() {
   total_kB=$(awk '/^MemTotal/ { print $2 }' /proc/meminfo)
   # tmpfs uses by default 50% of the available RAM, so the RAM should be at least
   # the double of the minimum tmpfs size required
-  RAM_NEEDED=$[ $DIB_MIN_TMPFS * 2 ]
+  RAM_NEEDED=$(($DIB_MIN_TMPFS * 2))
   [ $total_kB -lt $(($RAM_NEEDED*1024*1024)) ] || return 0
   echo "Not enough RAM to use tmpfs for build. ($total_kB < ${RAM_NEEDED}G)"
   return 1