This requires a special ramdisk that can set back the boot-args parameter in nvram.

Here's the Emergency Ramdisk

Please decompress the emergency ramdisk into iLiberty+ installation folder (normally C:\Program Files\iLiberty), you’ll have a file called Emergency.dmg in that folder. Then take the following steps:

1. Make sure phone is connected to computer, and is displaying “BSD Root: md0, major 2, minor 0″
2. Disconnect phone from computer
3. Power off phone
4. Press and hold Home button, connect phone to computer (don’t release Home yet), it’ll start booting
5. Wait till phone displays iTunes + USB cable icon, you may release Home now

Your phone is now in recovery mode. Now take the following steps:

6. Start iLiberty+, it should display Recovery Mode at the lower right corner of status bar. Goto Other Tools tab, select the Emergency.dmg into customized ramdisk, make sure Prepend Data is checked, then click Boot It, it will boot your phone back into normal mode.

More technical details about this issue can be found at: Here