include/linux/kconfig.h: ese macros which are already defined

It is better to use macros which are already available because then there
is just one location which needs to be change.

[akpm@linux-foundation.org: move IS_ENABLED definition to after the IS_BUILTIN and IS_MODULE definitions]
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>