The example in kernel_fdt.rst is inconsistently indented, making it
difficult to read.

Indent the example with the same standard as the other examples:
Four spaces for the ReST code block and for every nesting level.

Signed-off-by: J. Neuschäfer <j.ne@posteo.net>

Both SHA1 and (especially) MD5 are no longer as safe as they once were for
cryptographic use. Replaces examples which use them with examples using
SHA256 instead. This will provide more-secure defaults for users who use
documentation examples as a base for their own use. This is not too
necessary for non-verified-boot scenarios (since someone could just replace
the checksum), but I wanted to be complete.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Peter Robinson <pbrobinson@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>

Convert these to rST format and add them to the index.

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