stm32: Use local vars in stm32prog for initrd and fdt

Rather than assigning to the bootm_argv[] array multiple times, use
local variables for the two things that can change and assign them at
the end.

This makes it easier to drop the array eventually.

Tidu up an overly short line while we are here.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>