tools: open FIT image read-only

Open for reading as O_RDONLY instead of O_RDWR:
the only usage of the fd is for the single read() below;
this prevented
	mkimage -f auto -A arm64 \
		-T kernel -C lz4 -d Image-6.6.15.lz4 \
		-b mt8173-elm-hana-6.6.15.dtb outf
when the inputs were unwritable.

Link: https://bugs.debian.org/1063097


Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Reviewed-by: Dragan Simic <dsimic@manjaro.org>