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>

Documentation/admin-guide/bootconfig.rst

@@ -140,7 +140,9 @@ Since the boot configuration file is loaded with initrd, it will be added
 to the end of the initrd (initramfs) image file with padding, size,
 checksum and 12-byte magic word as below.
 
-[initrd][bootconfig][padding][size(u32)][checksum(u32)][#BOOTCONFIG\n]
+[initrd][bootconfig][padding][size(le32)][checksum(le32)][#BOOTCONFIG\n]
+
+The size and checksum fields are unsigned 32bit little endian value.
 
 When the boot configuration is added to the initrd image, the total
 file size is aligned to 4 bytes. To fill the gap, null characters