kconfig: generate lexer and parser during build instead of shipping

zconf.lex.c is generated by flex, zconf.tab.c by bison.  Instead of
running flex and bison during the kernel building, we conventionally
version-control those artifacts with _shipped suffix.

It is tedious to manually regenerate them every time we change the
real sources, zconf.l and zconf.y.

Remove the _shipped files and switch over to build-time generation
of the intermediate C files.

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