kconfig: remove unneeded buffer allocation in zconf_initscan()

In Kconfig, there is a stack to save the lexer state for each inclusion
level.

Currently, it operates as an empty stack, with the 'current_buf' always
pointing to an empty buffer. There is no need to preallocate the buffer.
Change it to a full stack.

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