scripts/make_fit: Move dtb processing into a function

scripts/make_fit: Move dtb processing into a function

Since build_fit() is getting quite long, move the dtb processing into a
separate function.

Change the double quotes in the write() call to single, to match the
rest of the script.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Nicolas Schier <nsc@kernel.org>
Reviewed-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Reviewed-by: Chen-Yu Tsai <wenst@chromium.org>
Link: https://patch.msgid.link/20260106162738.2605574-4-sjg@chromium.org
Signed-off-by: Nathan Chancellor <nathan@kernel.org>