kconfig: nconf: refactor attributes setup code

The current attributes setup code is strange; the array attribute[]
is set to values outside the range of the attribute_t enum.

At least,

  attributes_t attributes[ATTR_MAX+1] = {0};

... should be

  int attribute[ATTR_MAX+1] = {0};

Also, there is no need to hard-code the color-pair numbers in
attributes_t.

The current code is messy. Rewrite it.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>