configure: unify recursion into sub-Makefiles

Treat contrib/plugins the same as the firmware.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

configure

@@ -256,6 +256,7 @@ softmmu="yes"
 linux_user=""
 bsd_user=""
 plugins="$default_feature"
+subdirs=""
 ninja=""
 python=
 download="enabled"
@@ -1067,6 +1068,9 @@ if test "$static" = "yes" ; then
   fi
 fi
 test "$plugins" = "" && plugins=yes
+if test "$plugins" = "yes"; then
+  subdirs="$subdirs contrib/plugins"
+fi
 
 cat > $TMPC << EOF
 
@@ -1627,12 +1631,11 @@ done
 echo "# Automatically generated by configure - do not modify" > Makefile.prereqs
 
 # Mac OS X ships with a broken assembler
-roms=
 if have_target i386-softmmu x86_64-softmmu && \
         test "$targetos" != "darwin" && test "$targetos" != "sunos" && \
         test "$targetos" != "haiku" && \
         probe_target_compiler i386-softmmu; then
-    roms="pc-bios/optionrom"
+    subdirs="$subdirs pc-bios/optionrom"
     config_mak=pc-bios/optionrom/config.mak
     echo "# Automatically generated by configure - do not modify" > $config_mak
     echo "TOPSRC_DIR=$source_path" >> $config_mak
@@ -1641,7 +1644,7 @@ fi
 
 if have_target ppc-softmmu ppc64-softmmu && \
         probe_target_compiler ppc-softmmu; then
-    roms="$roms pc-bios/vof"
+    subdirs="$subdirs pc-bios/vof"
     config_mak=pc-bios/vof/config.mak
     echo "# Automatically generated by configure - do not modify" > $config_mak
     echo "SRC_DIR=$source_path/pc-bios/vof" >> $config_mak
@@ -1660,7 +1663,7 @@ if have_target s390x-softmmu && probe_target_compiler s390x-softmmu && \
       echo "WARNING: Your compiler does not support the z900!"
       echo "         The s390-ccw bios will only work with guest CPUs >= z10."
     fi
-    roms="$roms pc-bios/s390-ccw"
+    subdirs="$subdirs pc-bios/s390-ccw"
     config_mak=pc-bios/s390-ccw/config-host.mak
     echo "# Automatically generated by configure - do not modify" > $config_mak
     echo "SRC_PATH=$source_path/pc-bios/s390-ccw" >> $config_mak
@@ -1727,7 +1730,7 @@ if test "$container" != no; then
     echo "ENGINE=$container" >> $config_host_mak
     echo "RUNC=$runc" >> $config_host_mak
 fi
-echo "ROMS=$roms" >> $config_host_mak
+echo "SUBDIRS=$subdirs" >> $config_host_mak
 echo "PYTHON=$python" >> $config_host_mak
 echo "GENISOIMAGE=$genisoimage" >> $config_host_mak
 echo "MESON=$meson" >> $config_host_mak