config: allow using flash_lk2nd with heimdall too

One of the main purposes of lk2nd is to have a replacement for the stock bootloader "Odin mode" on many Samsung devices with sane fastboot implementation, which supports many more features like live booting and many others.

This is done by using heimdall to flash lk2nd to device's boot partition. Allow this method to be used with this flash action.

Example usage:

pmbootstrap flasher --method=heimdall-bootimg flash_lk2nd