kconfig: Drop IS_ENABLED() macro

We keep its definition in libpayload, though, to maintain compatibility
with existing payload code. For now.

Change-Id: I8fc0d0136ba2316ef393c5c17f2b3ac3a9c6328d
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/32230
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>

Documentation/getting_started/kconfig.md

@@ -1165,8 +1165,6 @@ saved .config file. As always, a 'select' statement overrides any specified
 - coreboot has added the glob operator '*' for the 'source' keyword.
 - coreboot’s Kconfig always defines variables except for strings. In other
   Kconfig implementations, bools set to false/0/no are not defined.
-- IS_ENABLED() is ‘false’ for undefined variables and ‘0’ variables. In Linux
-  (where the macro comes from) it’s ‘true’ as soon as the variable is defined.
 - coreboot’s version of Kconfig adds the KCONFIG_STRICT environment variable to
   error out if there are any issues in the Kconfig files.  In the Linux kernel,
   Kconfig will generate a warning, but will still output an updated .config or