IASL: Enable warnings as errors

We've actually got more warnings now than when I first tested IASL
warnings as errors.  Because of this, I'm adding it with the option
to have it disabled, in hopes that things won't get any worse as we
work on fixing the IASL warnings that are currently in the codebase.

- Enable IASL warnings as errors
- Disable warnings as errors in mainboards that currently have warnings.
- Print a really obnoxious message on those platforms when they build.
***** WARNING: IASL warnings as errors is disabled!  *****
*****          Please fix the ASL for this platform. *****

Change-Id: If0da0ac709bd8c0e8e2dbd3a498fe6ecb5500a81
Signed-off-by: Martin Roth <gaumless@gmail.com>
Reviewed-on: http://review.coreboot.org/10663
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

src/Kconfig

@@ -1067,6 +1067,19 @@ config WARNINGS_ARE_ERRORS
 	bool
 	default y
 
+# TODO: Remove this when all platforms are fixed.
+config IASL_WARNINGS_ARE_ERRORS
+	def_bool y
+	help
+	  Select to Fail the build if a IASL generates a warning.
+	  This will be defaulted to disabled for the platforms that
+	  currently fail.  This allows the REST of the platforms to
+	  have this check enabled while we're working to get those
+	  boards fixed.
+
+	  DO NOT ADD TO ANY ADDITIONAL PLATFORMS INSTEAD OF FIXING
+	  THE ASL.
+
 # The four POWER_BUTTON_DEFAULT_ENABLE, POWER_BUTTON_DEFAULT_DISABLE,
 # POWER_BUTTON_FORCE_ENABLE and POWER_BUTTON_FORCE_DISABLE options are
 # mutually exclusive. One of these options must be selected in the