cml1.bbclass: Add diffconfig task to cml1

cml1.bbclass: Add diffconfig task to cml1

diffconfig() is a new task that makes a diff between the
old and new config files and writes to the fragment.cfg result file.
menuconfig() always copy the original config file, so the user
doesn't need to copy it.

(From OE-Core rev: f0dcbbdcc37da12f84f730b8235f50be8c150c50)

Signed-off-by: João Henrique Ferreira de Freitas <joaohf@gmail.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>