kconfig: rename SYMBOL_AUTO to SYMBOL_NO_WRITE

Over time, the use of the flag SYMBOL_AUTO changed from initially
marking three automatically generated symbols ARCH, KERNELRELEASE and
UNAME_RELEASE to today's effect of protecting symbols from being
written out.

Currently, only symbols of type CHOICE and those with option
defconf_list set have that flag set.

Reflect that change in semantics in the flag's name.

Signed-off-by: Dirk Gouders <dirk@gouders.net>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>