mkimage: update man page and -h output

The man page correctly said that -B was ignored without -E, while the
`mkimage -h` output suggested otherwise. Now that -B can actually be
used by itself, update the man page.

While at it, also amend the `mkimage -h` line to mention the
connection with -E.

The FDT header is a fixed 40 bytes, so its size cannot (and is not)
modified, while its alignment is a property of the address in RAM one
loads the FIT to, so not something mkimage can affect in any way. (In
the file itself, the header is of course at offset 0, which has all
possible alignments already.)

Reported-by: Sean Anderson <seanga2@gmail.com>
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Reviewed-by: Simon Glass <sjg@chromium.org>