How do I "dd" a binary from a build script to a partition?

[Dragonink]

I am trying to use mkosi to build an image to use in a STM32MP board.

I (cross-)compile the bootloader in a build script and put its artifacts in $BUILDDIR.
Then I reserve a partition for it with repart:

[Partition]
Label=fsbl
Type=8DA63339-0007-60C0-C436-083AC8230908
SizeMinBytes=256K
SizeMaxBytes=256K

Now I need to "dd" the bootloader binary to that partition, but I don't know how.
Is there a way to do it with repart? Or do I need some additional scripts?

[Dragonink]

What I missed is that $DESTDIR is the image filesystem and repart has access to it.

I needed to:

1. Copy the artifact from $BUILDDIR to $DESTDIR/boot/fsbl.bin

2. truncate --size 256K "$DESTDIR/boot/fsbl.bin"

3. Edit the partition file:

   [Partition]
   Label=fsbl
   Type=8DA63339-0007-60C0-C436-083AC8230908
   SizeMinBytes=256K
   SizeMaxBytes=256K
   CopyBlocks=/boot/fsbl.bin