kernel-module-split.bbclass: support muti-lines in module conf

kernel-module-split.bbclass: support muti-lines in module conf

The expanded bitbake variables are raw strings so it does not support
writting multiple lines to a kernel module conf, add the support by
decoding it with 'unicode-escape'.

Reported-by: Mathias Thore <mathias.thore@atlascopco.com>
Signed-off-by: Ming Liu <liu.ming50@gmail.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>