Implement workaround for low-mem models.

The installer makes a backup (in memory) of itself to /bootfs, but after
restore to the new FAT32 partition doesn't remove that memory copy.
The 256MB model is so memory constrained that it fails, likely due to
an OOM problem. For those models (only), remove the RAM copy and when
the new boot partition gets mounted (to /rootfs/boot), make a
'mount --bind' to /bootfs, so if any code assumes the data/files to be
there, it'll still find them, so it won't break any code.

This fixes #520.

Signed-off-by: Diederik de Haas <github@cknow.org>

scripts/etc/init.d/rcS

@@ -1102,6 +1102,23 @@ sync
 umount /boot || fail
 log_msg_end "OK"
 
+model_memory=$(busybox free -m | grep "^Mem:" | awk '{print $2}')
+low_mem_model=""
+if [ "$model_memory" -le "260" ] ; then
+    low_mem_model="true"
+else
+    low_mem_model="false"
+fi
+log_msg "Model memory in MB: '$model_memory' is a low mem model: '$low_mem_model'"
+
+if [ "$low_mem_model" = "true" ] ; then
+    log_msg_start "Removing backup copy in /bootfs... "
+    rm -r -- /bootfs/*
+    log_msg_end "OK"
+else
+    log_msg "Keeping backup copy in /bootfs"
+fi
+
 # only do modprobe if the module is NOT built into the kernel
 if [ "$(grep -c $rootfstype /lib/modules/"$(uname -r)"/modules.builtin)" -eq 0 ]; then
     log_msg_start "Loading $rootfstype module... "
@@ -1120,6 +1137,12 @@ mkdir /rootfs/boot || fail
 mount $bootpartition /rootfs/boot || fail
 log_msg_end "OK"
 
+if [ "$low_mem_model" = "true" ] ; then
+    log_msg_start "Bind mount /rootfs/boot to /bootfs... "
+    mount --bind /rootfs/boot /bootfs
+    log_msg_end "OK"
+fi
+
 log_msg "Starting install process..."
 # With 'cdebootstrap_cmdline we're actually using the word splitting what SC2086 warns us against
 # This should probably be fixed at some point, but not in this run/branch.
@@ -1671,6 +1694,11 @@ chmod 0640 /rootfs/var/log/raspbian-ua-netinst.log
 
 log_msg_start "Unmounting filesystems... "
 
+if [ "$low_mem_model" = "true" ] ; then
+    log_msg_inline "removing bind mount to /bootfs... "
+    umount /bootfs
+fi
+
 umount /rootfs/boot
 umount /rootfs
 log_msg_end "OK"