mkimage: Use board config to get CONFIG_FIT_SIGNATURE value

The value of this config variable is not available to image.h on the host,
since the board config is not actually included. Bring this in so that
mkimage will be built with image-signing support for sandbox at least.
Signed-off-by: NSimon Glass <sjg@chromium.org>

config.mk

@@ -213,6 +213,10 @@ endif
 # TODO(sjg@chromium.org): Is this correct on Mac OS?
 ifdef CONFIG_FIT_SIGNATURE
 HOSTLIBS	+= -lssl -lcrypto
+
+# This affects include/image.h, but including the board config file
+# is tricky, so manually define this options here.
+HOSTCFLAGS	+= -DCONFIG_FIT_SIGNATURE
 endif
 
 ifneq ($(CONFIG_SYS_TEXT_BASE),)