kconfig: Update IS_ENABLED() internals

kconfig: Update IS_ENABLED() internals

The config_enabled() macro currently uses 0 as the default value. Update
it to allow any value, so we can pass it something else, such as a
non-existent function, to produce a build error if it is not defined.

Also tidy up the code style for IS_ENABLED() and drop the unnecessary
brackets (the value is a simple 0 or 1).

Signed-off-by: Simon Glass <sjg@chromium.org>