Sure a third party app or tool will do the job, but it is good to learn something.....
Backuping efs (in this device it is "modemst") partition is an essential step to keep it safe after rooting since a bad flashing/modding of your Xperia L may result in a corruption of that partition. so it is better to be safe than sorry, so any disturbance in the EFS data might result in null IMEI and baseband version.
This will backup data like device specific essential information such as the IMEI (International Mobile Station Equipment Identity), wireless device MAC address, baseband version, product code, system ID and NV data.
You must have a proper root access in addition to busybox installed (preferred)
Do this step after rooting.
You can dump these partitions also using adb shell command. but here i will show you how to backup using a terminal emulator.
How to do it
Download "terminal emulator" app from play store and install it.
Open the app and type (DONT MAKE MISTAKES!):
copy images from internal sdcard to a safe place such as pc.Code:dd if=/dev/block/platform/msm_sdcc.1/by-name/modemst1 of=/sdcard/efs1.img dd if=/dev/block/platform/msm_sdcc.1/by-name/modemst2 of=/sdcard/efs2.img
Note that the size of .img must be as follow: modemst1/2 (8MB each)
Make sure Backuped images are in your internal sdcard (not in any folder).
open terminal emulator and type (DONT MAKE MISTAKES!):
Code:su dd if=/sdcard/efs1.img of=/dev/block/platform/msm_sdcc.1/by-name/modemst1 dd if=/sdcard/efs2.img of=/dev/block/platform/msm_sdcc.1/by-name/modemst2