configure: Fix code which creates config.mak files

These files are created by configure and grow
unnecessarily at each new call of configure:

roms/seabios/config.mak
roms/vgabios/config.mak
libhw32/config.mak
libhw64/config.mak

libhw32/config.mak and libhw64/config.mak set
compiler options, and the wrong old code results
in very long command lines.

The new code always writes a new config.mak
instead of appending to an existing one.
Signed-off-by: NStefan Weil <weil@mail.berlios.de>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

configure

@@ -2702,7 +2702,7 @@ fi
 # temporary config to build submodules
 for rom in seabios vgabios ; do
     config_mak=roms/$rom/config.mak
-    echo "# Automatically generated by configure - do not modify" >> $config_mak
+    echo "# Automatically generated by configure - do not modify" > $config_mak
     echo "SRC_PATH=$source_path/roms/$rom" >> $config_mak
     echo "CC=$cc" >> $config_mak
     echo "BCC=bcc" >> $config_mak
@@ -2718,7 +2718,7 @@ for hwlib in 32 64; do
   mkdir -p $d
   rm -f $d/Makefile
   ln -s $source_path/Makefile.hw $d/Makefile
-  echo "QEMU_CFLAGS+=-DTARGET_PHYS_ADDR_BITS=$hwlib" >> $d/config.mak
+  echo "QEMU_CFLAGS+=-DTARGET_PHYS_ADDR_BITS=$hwlib" > $d/config.mak
 done
 
 d=libuser