bootm: Reduce arguments to boot_get_ramdisk()

bootm: Reduce arguments to boot_get_ramdisk()

This function normally only uses one argument so pass it in directly.
Move comments to the header file so could one day include these
functions in API docs. Fix up the u8 argument while here, since it
avoids the compiler having to mask the value on some machines.

The Android case here is bit strange, since it can use argv[0], so deal
with that in the caller.

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