configure: Display if libfdt is from system or git

The configure script outputs "yes" regardless which libfdt is used:

  ./configure
  [...]
  fdt support       yes

Sometimes you can have both system and local git version available,
change the configure script to display which library got selected:

  debian8$ dpkg-query --showformat='${Version}\n' --show libfdt-dev
  1.4.0+dfsg-1

  debian8$ ./configure
  [...]
  fdt support       git
Signed-off-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20180415230522.24404-3-f4bug@amsat.org>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

configure

@@ -3789,15 +3789,14 @@ int main(void) { fdt_first_subnode(0, 0); return 0; }
 EOF
   if compile_prog "" "$fdt_libs" ; then
     # system DTC is good - use it
-    fdt=yes
+    fdt=system
   else
       # have GIT checkout, so activate dtc submodule
       if test -e "${source_path}/.git" ; then
           git_submodules="${git_submodules} dtc"
       fi
       if test -d "${source_path}/dtc/libfdt" || test -e "${source_path}/.git" ; then
-          fdt=yes
-          dtc_internal="yes"
+          fdt=git
           mkdir -p dtc
           if [ "$pwd_is_source_path" != "y" ] ; then
               symlink "$source_path/dtc/Makefile" "dtc/Makefile"
@@ -6338,7 +6337,7 @@ fi
 if test "$preadv" = "yes" ; then
   echo "CONFIG_PREADV=y" >> $config_host_mak
 fi
-if test "$fdt" = "yes" ; then
+if test "$fdt" != "no" ; then
   echo "CONFIG_FDT=y" >> $config_host_mak
 fi
 if test "$membarrier" = "yes" ; then
@@ -7140,7 +7139,7 @@ echo "QEMU_CFLAGS+=$cflags" >> $config_target_mak
 
 done # for target in $targets
 
-if [ "$dtc_internal" = "yes" ]; then
+if [ "$fdt" = "git" ]; then
   echo "config-host.h: subdir-dtc" >> $config_host_mak
 fi
 if [ "$capstone" = "git" -o "$capstone" = "internal" ]; then