configure: do not create roms/seabios/config.mak if SeaBIOS not present

If roms/seabios/Makefile is not present, the configure script
is not creating the roms/seabios directory anymore (commit
5dce7b8d8c, "configure: remove DIRS", 2021-12-18); thus, creating
roms/seabios/config.mak fails.

The easiest thing to do is to not create the file, since it will not
be used.

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

configure

@@ -3704,7 +3704,8 @@ export target_list source_path use_containers cpu
 $source_path/tests/tcg/configure.sh)
 
 # temporary config to build submodules
-for rom in seabios; do
+if test -f $source_path/roms/seabios/Makefile; then
+  for rom in seabios; do
     config_mak=roms/$rom/config.mak
     echo "# Automatically generated by configure - do not modify" > $config_mak
     echo "SRC_PATH=$source_path/roms/$rom" >> $config_mak
@@ -3717,7 +3718,8 @@ for rom in seabios; do
     echo "IASL=$iasl" >> $config_mak
     echo "LD=$ld" >> $config_mak
     echo "RANLIB=$ranlib" >> $config_mak
-done
+  done
+fi
 
 config_mak=pc-bios/optionrom/config.mak
 echo "# Automatically generated by configure - do not modify" > $config_mak