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Typically, the unsigned bootable image is generated and programmed to the destination memory during the development phase.
The elftosb utility supports unsigned bootable image generation using options, BD file, and ELF/SREC file generated by toolchain.
Taking the Flashloader project as an example, here are the steps to create a bootable image for Flashloader.

Step 1: Create a BD file. For unsigned image creation, the “constants” block is optional, as shown below.

options {
    flags = 0x00;
    startAddress = 0x200000000x60000000;
    ivtOffset = 0x400;
    initialLoadSize = 0x2000;
    entryPointAddress = 0x60002411;
}
sources {
    elfFile = extern(0);
}
section (0)
{
}

After the BD file is created, place it into the same folder that holds elftosb utility executable.

Step 2: Copy Flashloader.srec provided in the release package into the same folder that holds elftosb utility executable.

Step 3: Generate the Bootable image using elftosb utility.

elftosb.exe -f imx -V -c imx-unsigned.bd -o ivt_flashloader_unsigned.bin flashloader.srec

Then, there are two bootable images generated by elftosb utility. The first one is ivt_flashloader_unsigned.bin. The memory regions from 0 to ivt_offset are filled with padding bytes (all 0x00s).
The second one is ivt_flashloader_nopadding.bin, which starts from ivtdata directly without any padding before ivt.