configure: Allow builds without any system or user emulation

The old code aborted configure when no emulation target was selected.
Even after removing the 'exit 1', it tried to read from STDIN
when QEMU was configured with

    configure' '--disable-user' '--disable-system'

This is fixed here.
Signed-off-by: NStefan Weil <sw@weilnetz.de>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

Makefile

@@ -52,8 +52,13 @@ SUBDIR_MAKEFLAGS=$(if $(V),,--no-print-directory) BUILD_DIR=$(BUILD_DIR)
 SUBDIR_DEVICES_MAK=$(patsubst %, %/config-devices.mak, $(TARGET_DIRS))
 SUBDIR_DEVICES_MAK_DEP=$(patsubst %, %/config-devices.mak.d, $(TARGET_DIRS))
 
+ifeq ($(SUBDIR_DEVICES_MAK),)
+config-all-devices.mak:
+	$(call quiet-command,echo '# no devices' > $@,"  GEN   $@")
+else
 config-all-devices.mak: $(SUBDIR_DEVICES_MAK)
 	$(call quiet-command,cat $(SUBDIR_DEVICES_MAK) | grep =y | sort -u > $@,"  GEN   $@")
+endif
 
 -include $(SUBDIR_DEVICES_MAK_DEP)
 

configure

@@ -1286,10 +1286,6 @@ if test -z "$target_list" ; then
 else
     target_list=`echo "$target_list" | sed -e 's/,/ /g'`
 fi
-if test -z "$target_list" ; then
-    echo "No targets enabled"
-    exit 1
-fi
 # see if system emulation was really requested
 case " $target_list " in
   *"-softmmu "*) softmmu=yes