kconfig: remove keyword lookup table entirely

Commit 7a88488b ("[PATCH] kconfig: use gperf for kconfig keywords")
introduced gperf for the keyword lookup.

Then, commit bb3290d9 ("Remove gperf usage from toolchain") killed
the gperf use. As a result, the linear keyword search was left behind.

If we do not use gperf, there is no reason to have the separate table
of the keywords. Move all keywords back to the lexer.

I also refactored the lexer to remove the COMMAND and PARAM states.

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