kconfig: Add host support to CONFIG_IS_ENABLED()

At present we must separately test for the host build for many options,
since we force them to be enabled. For example, CONFIG_FIT is always
enabled in the host tools, even if CONFIG_FIT is not enabled by the
board itself.

It would be more convenient if we could use, for example,
CONFIG_IS_ENABLED(FIT) and get CONFIG_HOST_FIT, when building for the
host. Add support for this.

Signed-off-by: Simon Glass <sjg@chromium.org>
Suggested-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk> # b4f73886