mx6qsabrelite: add helper for read and reset boot counter

Each board may have it's own method and place to store the boot counter
related variables.
With `/etc/default/rollback_helper` file it is possible to
define board-specific functions allowing to read and correctly
reset variables.

This patch adds helper for boards using u-boot tools to access
U-Boot environment.

Signed-off-by: Denis Pynkin <denis.pynkin@collabora.com>

scripts/setup-u-boot-tools.sh

@@ -73,3 +73,21 @@ cat > $sysroot/etc/fw_env.config <<E_O_F
 # MTD device name   Device offset   Env. size   Flash sector size
 $DEVICE    $OFFSET    $ENVSIZE    $SECSIZE
 E_O_F
+
+cat > $sysroot/etc/default/rollback_helper <<"E_O_F"
+# Helper for rollback script
+# For U-Boot -- u-boot-tools must be configured in "/etc/fw_env.config"
+
+# Return the value of requested variable.
+# U-Boot bootcount mechanism is working only if "upgrade_available" is set.
+get_variable(){
+    fw_printenv -n upgrade_available >/dev/null 2>&1 && \
+        fw_printenv -n "$1" 2>/dev/null || echo 0
+}
+
+# Reset the counter for U-Boot.
+reset_counter(){
+    fw_setenv upgrade_available
+    fw_setenv bootcount
+}
+E_O_F