Replace includes of build.h with version.h

As build.h is an auto-generated file it was necessary to add it as
an explicit prerequisite in the Makefiles. When this was forgotten
abuild would sometimes fail with following error:

   fatal error: build.h: No such file or directory

Fix this error by compiling version.c into all stages.

Change-Id: I342f341077cc7496aed279b00baaa957aa2af0db
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: http://review.coreboot.org/7510
Tested-by: build bot (Jenkins)
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>

src/lib/version.c

@@ -40,3 +40,13 @@ const char coreboot_compile_time[]   = COREBOOT_COMPILE_TIME;
 const char coreboot_compile_by[]     = COREBOOT_COMPILE_BY;
 const char coreboot_compile_host[]   = COREBOOT_COMPILE_HOST;
 const char coreboot_compile_domain[] = COREBOOT_COMPILE_DOMAIN;
+
+const char coreboot_dmi_date[]       = COREBOOT_DMI_DATE;
+
+const struct bcd_date coreboot_build_date = {
+	.century = 0x20,
+	.year = COREBOOT_BUILD_YEAR_BCD,
+	.month = COREBOOT_BUILD_MONTH_BCD,
+	.day = COREBOOT_BUILD_DAY_BCD,
+	.weekday = COREBOOT_BUILD_WEEKDAY_BCD,
+};