tools: use read-only mmap in fit_check_sign

Add an option to open files in read-only mode in mmap_fdt so
that fit_check_sign can be used to inspect files on read-only
filesystems.
For example, this is useful when a key is shipped in a read-only
rootfs or squashfs.

Signed-off-by: Luca Boccassi <luca.boccassi@microsoft.com>