BBHandler/ConfHandler: Improve multiline comment handling

Faced with an expression like:

 # Some comment \
 FOO = "bar"

what should bitbake do? Technically, the \ character means its multiline and
currently the code treats this as a continuation of the comment. This can
surprise some people and is not intuitive.

This patch makes bitbake simply error and asks the user to be clearer
about what they mean.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>