Add comment for NOHDD which is used for skipping building the HDDIMG if
set to 1
(From OE-Core master rev: 9843ad9d783f68b97fedfe5b435528538bb26c1c)
(From OE-Core rev: 3c5145670cff69828e7adbce6bde2ec8ddc60e84)
Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>