bootm: Pass image buffer to boot_get_fdt()

bootm: Pass image buffer to boot_get_fdt()

Rather than having boot_get_fdt() calculate this, move the calculation
into the caller. This removes the access to argv[0] in this function,
so we can later refactor it to just accept argv[2] instead of the whole
argv[].

Move the function comment to the header file and fix the u8 argument,
while we are here.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>