kconfig: remove grammatically ambiguous option_error

This commit decreases 6 shift/reduce conflicts, and finally achieves
conflict-free parser.

Since Kconfig has no terminator for a config block, detecting the end
of config_stmt is not easy.

For example, there are two ways for handling the error in the following
code:

  1 config FOO
  2         =

 [A] Print "unknown option" error, assuming the line 2 is a part of
     config_option_list

 [B] Print "invalid statement", assuming the line 1 is reduced into
     a config_stmt by itself

Bison actually chooses [A] because it performs the shift rather than
the reduction where both are possible.

However, there is no reason to choose one over the other.

Let's remove the option_error, and let it fall back to [B].

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>