kwbimage: Make the Makefile pass in CONFIG_SYS_SPI_U_BOOT_OFFS

We can't use config.h directly as some platforms include headers that
aren't safe to use in normal Linux userland.

Signed-off-by: Tom Rini <trini@ti.com>
diff --git a/tools/Makefile b/tools/Makefile
index 6e1ce79..e4b23eb 100644
--- a/tools/Makefile
+++ b/tools/Makefile
@@ -115,6 +115,10 @@
 HOST_EXTRACFLAGS	+= -DCONFIG_FIT_SIGNATURE
 endif
 
+ifdef CONFIG_SYS_SPI_U_BOOT_OFFS
+HOSTCFLAGS_kwbimage.o += -DCONFIG_SYS_SPI_U_BOOT_OFFS=$(CONFIG_SYS_SPI_U_BOOT_OFFS)
+endif
+
 # MXSImage needs LibSSL
 ifneq ($(CONFIG_MX23)$(CONFIG_MX28)$(CONFIG_FIT_SIGNATURE),)
 HOSTLOADLIBES_mkimage += -lssl -lcrypto