You can't create a boot.wim file from an install.wim. Boot.wim files are created with the Windows Automated Installation Kit. While a boot.wim file can be created from scratch using just the WAIK alone, you're better off using WAIK+MDT. Use the Microsoft Deployment Toolkit in conjuction with the WAIK tools to build a boot disc painlessly, as you're really not supposed to use WAIK all by itself.
Once you have a boot wim, use WDS to boot to the boot.wim, which will pull the install.wim from the deployment share you build in MDT 2013.
There is mass confusion on this issue all around, so please try to remember that boot.wims are created from the WAIK, and you use MDT to push the install.wims.
How to Use MDT 2013 to build a boot image to install Windows 7 from WDS
Install MDT 2013 and ADK 8.1
Download & Install MDT 2013 Update 1 -
Download ADK 8.1 -
Create Deployment Share -
Import Drivers -
Upload boot image from MDT into WDS -
It takes time, and some real work, but MDT is by far the best way to build boot.wim files. Build your install.wim files in a VM enviroment, they'll be "hardware independent" and will run on everything regardless of make or model.