kconfig/streamline-config.pl: Simplify backslash line concatination

Simplify the way lines ending with backslashes (continuation) in Makefiles
is parsed. This is needed to implement a necessary fix.

Tested-by: Thomas Lange <thomas-lange2@gmx.de>
Cc: stable@vger.kernel.org
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>