linux/kconfig.h: create two- and three-argument versions of CONFIG_IS_ENABLED

linux/kconfig.h: create two- and three-argument versions of CONFIG_IS_ENABLED

This adds a bunch of preprocessor magic to extend the capabilities of
CONFIG_IS_ENABLED. The existing semantics of

  CONFIG_IS_ENABLED(FOO)

expanding to a 1 or 0 (depending on build context and the defined-ness
or not of the appropriate CONFIG_FOO/CONFIG_SPL_FOO/CONFIG_TPL_FOO)
are of course preserved. With this, one is also allowed a two-argument
form

  CONFIG_IS_ENABLED(FOO, (something))

which expands to something precisely when CONFIG_IS_ENABLED(FOO) would
expand to 1, and expands to nothing otherwise. It is, in other words,
completely equivalent to the three lines

  #if CONFIG_IS_ENABLED(FOO)
  something
  #endif

The second argument must be parenthesized in order to allow any
tokens, including a trailing comma, to appear - one use case for this
is precisely to make it a bit more ergonomic to build an array and
only include certain items depending on .config. That should increase
both readability and not least "git grep"ability.

A third variant is also introduced,

  CONFIG_IS_ENABLED(FOO, (xxx), (yyy))

which corresponds to

  #if CONFIG_IS_ENABLED(FOO)
  xxx
  #else
  yyy
  #endif

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>