uboot-imx

binman: Support updating all device tree files

Binman currently supports updating the main device tree with things like
the position of each entry. Extend this support to SPL and TPL as well,
since they may need (a subset of) this information.

Also adjust DTB output files to have a .out extension since this seems
clearer than having a .dtb extension with 'out' in the name somwhere.

Also add a few missing comments and update the DT setup code to use
ReadFile and WriteFile().

Signed-off-by: Simon Glass <sjg@chromium.org>