docs: bootconfig: Add the endianness of fields

Add a description about the endianness of the size and the checksum
fields. Those must be stored as le32 instead of u32. This will allow
us to apply bootconfig to the cross build initrd without caring
the endianness.

Link: https://lkml.kernel.org/r/160583936246.547349.10964204130590955409.stgit@devnote2

Reported-by: Steven Rostedt <rostedt@goodmis.org>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>