package/boost: fix silent build

When doing a build with "make -s" (silent option), the build of Boost
fails. Indeed, in this case boost.mk is passing -d as a option to
Boost, but -d must be followed by a digit.

Fixes:
 - https://bugs.buildroot.org/show_bug.cgi?id=14951

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/boost/boost.mk

@@ -128,8 +128,8 @@ BOOST_WITHOUT_FLAGS_COMMASEPARATED += $(subst $(space),$(comma),$(strip $(BOOST_
 BOOST_FLAGS += $(if $(BOOST_WITHOUT_FLAGS_COMMASEPARATED), --without-libraries=$(BOOST_WITHOUT_FLAGS_COMMASEPARATED))
 
 # how verbose should the build be?
-BOOST_OPTS += $(if $(QUIET),-d,-d+1)
-HOST_BOOST_OPTS += $(if $(QUIET),-d,-d+1)
+BOOST_OPTS += $(if $(QUIET),-d0,-d+1)
+HOST_BOOST_OPTS += $(if $(QUIET),-d0,-d+1)
 
 define BOOST_CONFIGURE_CMDS
 	cd $(@D) && ./bootstrap.sh $(BOOST_FLAGS)