BUILD: makefile: sort the features list

The features list that appears in -vv appears in a random order, which
always makes it a pain to look for certain features. Let's sort it.
diff --git a/include/make/options.mk b/include/make/options.mk
index 4179e9a..db4502c 100644
--- a/include/make/options.mk
+++ b/include/make/options.mk
@@ -23,7 +23,7 @@
 # Make a list of all known features with +/- prepended depending on their
 # activation status. Must be a macro so that dynamically enabled ones are
 # evaluated with their current status.
-build_features  = $(foreach opt,$(patsubst USE_%,%,$(use_opts)),$(if $(USE_$(opt)),+$(opt),-$(opt)))
+build_features  = $(foreach opt,$(patsubst USE_%,%,$(sort $(use_opts))),$(if $(USE_$(opt)),+$(opt),-$(opt)))
 
 # This returns a list of -DUSE_* for all known USE_* that are set
 opts_as_defines = $(foreach opt,$(use_opts),$(if $($(opt)),-D$(opt),))