kconfig: error out if a recursive variable references itself

kconfig: error out if a recursive variable references itself

When using a recursively expanded variable, it is a common mistake
to make circular reference.

For example, Make terminates the following code:

  X = $(X)
  Y := $(X)

Let's detect the circular expansion in Kconfig, too.

On the other hand, a function that recurses itself is a commonly-used
programming technique.  So, Make does not check recursion in the
reference with 'call'.  For example, the following code continues
running eternally:

  X = $(call X)
  Y := $(X)

Kconfig allows circular expansion if one or more arguments are given,
but terminates when the same function is recursively invoked 1000 times,
assuming it is a programming mistake.

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