cmake: fixing toolchain_parse_make_rule to correctly handle `\` files

# The argument 'include_files' is an output parameter with the result

# of parsing the include files.

function(toolchain_parse_make_rule input_file include_files)

  file(READ ${input_file} input)

  file(STRINGS ${input_file} input)

  # The file is formatted like this:

  # empty_file.o: misc/empty_file.c \

  # nrf52840dk_nrf52840/nrf52840dk_nrf52840.dts \

  # nrf52840_qiaa.dtsi

  # Get rid of the backslashes

  string(REPLACE "\\" ";" input_as_list ${input})

  # The dep file will contain `\` for line continuation.

  # This results in `\;` which is then treated a the char `;` instead of

  # the element separator, so let's get the pure `;` back.

  string(REPLACE "\;" ";" input_as_list ${input})

  # Pop the first line and treat it specially

  list(GET input_as_list 0 first_input_line)

  list(POP_FRONT input_as_list first_input_line)

  string(FIND ${first_input_line} ": " index)

  math(EXPR j "${index} + 2")

  string(SUBSTRING ${first_input_line} ${j} -1 first_include_file)

  list(REMOVE_AT input_as_list 0)

  list(APPEND result ${first_include_file})

  # Remove whitespace before and after filename and convert to CMake path.

  string(STRIP "${first_include_file}" first_include_file)

  file(TO_CMAKE_PATH "${first_include_file}" first_include_file)

  set(result "${first_include_file}")

  # Add the other lines

  list(APPEND result ${input_as_list})

  # Strip away the newlines and whitespaces

  list(TRANSFORM result STRIP)

  # Remove whitespace before and after filename and convert to CMake path.

  foreach(file ${input_as_list})

    string(STRIP "${file}" file)

    file(TO_CMAKE_PATH "${file}" file)

    list(APPEND result "${file}")

  endforeach()

  set(${include_files} ${result} PARENT_SCOPE)

endfunction()