Introduce halt()

It's a portable and generic way to halt the system.
Useful when waiting for the platform to reset.

Change-Id: Ie07f3333d294a4d3e982cbc2ab9014c94b39fce0
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: http://review.coreboot.org/7605
Tested-by: build bot (Jenkins)
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>

src/lib/Makefile.inc

@@ -104,6 +104,11 @@ $(obj)/lib/version.smm.o : $(obj)/build.h
 romstage-y += bootmode.c
 ramstage-y += bootmode.c
 
+bootblock-y += halt.c
+romstage-y += halt.c
+ramstage-y += halt.c
+smm-y += halt.c
+
 ifeq ($(CONFIG_RELOCATABLE_MODULES),y)
 ramstage-y += rmodule.c
 romstage-$(CONFIG_RELOCATABLE_RAMSTAGE) += rmodule.c