cbfstool: Unset ${DEBUG} when making vboot hostlib

Vboot's Makefile is controlled by a ${DEBUG} environment variable.
As the name is very generic, it may be set by accident without any
intention to change the build. Having it set would break reproduci-
bility at least but it also turns out that the hostlib build would
be incomplete so that linking cbfstool fails due to internal calls
to vb2api_fail() which is not built in.

Change-Id: I2a9eb9a645c70451a320c455b8f24bfed197117c
Signed-off-by: Nico Huber <nico.huber@secunet.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/55764
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>

util/cbfstool/Makefile.inc

@@ -135,6 +135,7 @@ $(VBOOT_HOSTLIB):
 		CC="$(HOSTCC)" \
 		PKG_CONFIG="true" \
 		V=$(V) \
+		DEBUG= \
 		hostlib
 
 $(objutil)/cbfstool/%.o: $(objutil)/cbfstool/%.c