[grub]: Allow ONiE oneshoot boot for FW update. (sonic-net#3935)

Added ONiE oneshoot boot feature

Signed-off-by: Nazarii Hnydyn <nazariig@mellanox.com>

installer/x86_64/install.sh

@@ -555,15 +555,21 @@ EOF
 # Add the logic to support grub-reboot and grub-set-default
 cat <<EOF >> $grub_cfg
 if [ -s \$prefix/grubenv ]; then
-  load_env
+    load_env
 fi
-if [ "\${saved_entry}" ] ; then
-   set default="\${saved_entry}"
+if [ "\${saved_entry}" ]; then
+    set default="\${saved_entry}"
 fi
-if [ "\${next_entry}" ] ; then
-   set default="\${next_entry}"
-   set next_entry=
-   save_env next_entry
+if [ "\${next_entry}" ]; then
+    set default="\${next_entry}"
+    unset next_entry
+    save_env next_entry
+fi
+if [ "\${onie_entry}" ]; then
+    set next_entry="\${default}"
+    set default="\${onie_entry}"
+    unset onie_entry
+    save_env onie_entry next_entry
 fi
 
 EOF