arch/x86/Makefile.inc: Calculate CBFS_BASE_ADDRESS variable

The CBFS_BASE_ADDRESS can be compared against values used with cbfstool
to generate warnings.  This can help cut down on mistakes and debug
time.

Change-Id: I149007dd637661f799a0f2cdb079d11df726ca86
Signed-off-by: Martin Roth <gaumless@gmail.com>
Reviewed-on: http://review.coreboot.org/10681
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
diff --git a/src/arch/x86/Makefile.inc b/src/arch/x86/Makefile.inc
index 08246044..2448590 100644
--- a/src/arch/x86/Makefile.inc
+++ b/src/arch/x86/Makefile.inc
@@ -65,6 +65,9 @@
 CBFSTOOL_PRE_OPTS  = -b $(shell cat $(objcbfs)/base_xip.txt) -S ".car.data"
 endif
 
+## Calculate the base address of CBFS for later comparisons
+CBFS_BASE_ADDRESS=$(call int-add, $(call int-subtract, 0xffffffff $(CONFIG_CBFS_SIZE)) 1)
+
 ###############################################################################
 # bootblock
 ###############################################################################