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kconfig: fix ambiguous grammar in terms of new lines
This commit decreases 8 shift/reduce conflicts.
A certain amount of grammatical ambiguity comes from how to reduce
excessive T_EOL tokens.
Let's take a look at the example code below:
1 config A
2 bool "a"
3
4 depends on B
5
6 config B
7 def_bool y
The line 3 is melt into "config_option_list", but the line 5 can be
either a part of "config_option_list" or "common_stmt" by itself.
Currently, the lexer converts '\n' to T_EOL verbatim. In Kconfig,
a new line works as a statement terminator, but new lines in empty
lines are not critical since empty lines (or lines that contain only
whitespaces/comments) are just no-op.
If the lexer simply discards no-op lines, the parser will not be
bothered by excessive T_EOL tokens.
Of course, this means we are shifting the complexity from the parser
to the lexer, but it is much easier than tackling on shift/reduce
conflicts.
I introduced the second stage lexer to tweak the behavior.
Discard T_EOL if the previous token is T_EOL or T_HELPTEXT.
Two T_EOL tokens in a row is meaningless. T_HELPTEXT is a special
token that is reduced without T_EOL.
Signed-off-by: 
Masahiro Yamada <yamada.masahiro@socionext.com>