tools: moveconfig: fix cleanup of defines across multiple lines

tools: moveconfig: fix cleanup of defines across multiple lines

Correct the clean-up of such defines that continue across multiple
lines, like follows:

  #define CONFIG_FOO "this continues to the next line " \
          "this line should be removed too" \
          "this line should be removed as well"

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Tom Rini <trini@konsulko.com>