Thanks guy it works on switching on my Linksys WRT 1900AC V2.
Also after booting to stock and go to Administration.. and restore to previous firmware boots to another partition without Reset.
Stock Linksys WRT 1900AC V2 Boot to 2nd Partition from Stock Firmware- go to Admin.. and click restore to previous firmware.It will boot to another partition whatever you have flashed.
OpenWrt-To check Active Boot Partition type in SSH Puty to your router it will show 1 or 2.
/usr/sbin/fw_printenv -n boot_part
Boot to Alternate partition type 1 or 2
For Stock type 1
/usr/sbin/fw_setenv boot_part 1
For Custom Firmware Type 2
/usr/sbin/fw_setenv boot_part 2
I hope it will Help People.