Improve free space checks

And don't count as needed space that will be freed from the replaced
files. This will allow to flash the zip over an existing installation
when space is tight.

Also:
awk      -->  tr | cut  because recovery might not provide awk
grep -v  -->  tail -1   in du because it might spit out 3 lines

build/meta/com/google/android/update-binary

@@ -30,6 +30,12 @@ cleanup() {
   umount -l "$SYSTEM_MNT"
 }
 
+error_no_space() {
+  ui_print "Not enough space for GApps! Aborting"
+  cleanup
+  exit 1
+}
+
 get_block_for_mount_point() {
   grep -v "^#" /etc/recovery.fstab | grep " $1 " | tail -n1 | tr -s ' ' | cut -d' ' -f1
 }
@@ -61,6 +67,20 @@ find_block() {
   fi
 }
 
+compute_apps_size() {
+  NEEDED_STORAGE_SYSTEM=$(expr $(du -cs `find -maxdepth 1 -mindepth 1` | tail -n1 | cut -f1) + $STORAGE_BUFFER)
+  RECLAIMABLE_STORAGE_SYSTEM=$(find . -type f | sed "s|^./|$SYSTEM_OUT/|" | xargs ls -d 2>/dev/null | xargs du -cs PLACEHOLDER 2>/dev/null | tail -n1 |  cut -f1)
+  NEEDED_STORAGE_SYSTEM=$(expr $NEEDED_STORAGE_SYSTEM - $RECLAIMABLE_STORAGE_SYSTEM)
+}
+
+remove_big_optional_apps() {
+  ui_print "Low resource device detected, removing large extras"
+  rm -rf app/MarkupGoogle
+  rm -rf priv-app/AndroidMigratePrebuilt
+  rm -rf priv-app/SetupWizardPrebuilt
+  rm -rf priv-app/Velvet
+}
+
 ui_print "**********************"
 ui_print "MindTheGapps installer"
 ui_print "**********************"
@@ -105,9 +125,8 @@ exit 1
 fi
 SYSTEM_OUT="${SYSTEM_MNT}/system"
 
-LOWMEM=1572864
-MEM=`grep MemTotal /proc/meminfo | awk '{ print $2 }'`
-STORAGE=`df $SYSTEM_MNT | grep -v Filesystem | awk '{ print $4 }'`
+# Compute storage requirements
+SYSTEM_STORAGE=`df $SYSTEM_MNT | tail -1 | tr -s ' ' | cut -d ' ' -f4`
 STORAGE_BUFFER=10240
 
 ui_print "Extracting files"
@@ -115,21 +134,13 @@ cd /tmp
 unzip -o "$ZIP"
 rm -rf META-INF
 cd system
+compute_apps_size
 
-NEEDED_STORAGE=`expr $(du -s . | awk '{ print $1 }') + $STORAGE_BUFFER`
-
-if [ "$MEM" -lt "$LOWMEM" ] || [ "$STORAGE" -lt "$NEEDED_STORAGE" ]; then
-  ui_print "Low resource device detected, removing large extras"
-  rm -rf app/MarkupGoogle
-  rm -rf priv-app/AndroidMigratePrebuilt
-  rm -rf priv-app/SetupWizardPrebuilt
-  rm -rf priv-app/Velvet
-  NEEDED_STORAGE=`expr $(du -s . | awk '{ print $1 }') + $STORAGE_BUFFER`
-  if [ "$STORAGE" -lt "$NEEDED_STORAGE" ]; then
-    ui_print "Not enough space for GApps! Aborting"
-    cd ..
-    rm -rf system
-    exit 1
+if [ "$SYSTEM_STORAGE" -lt "$NEEDED_STORAGE_SYSTEM" ]; then
+  remove_big_optional_apps
+  compute_apps_size
+  if [ "$SYSTEM_STORAGE" -lt "$NEEDED_STORAGE_SYSTEM" ]; then
+    error_no_space
   fi
 fi