Deal with "sed -i" portability

An empty extension for sed -i is not widely available.

* Where it isn't available (eg. macOS's sed) "sed -i -e" will
  create a backup file with the "-e" suffix.

* Even GNU sed documentation says it's "not recommended".

This commit deals with it by:

* Replace it with a more appropriate tool (kconfig-tweak)

* Or, specify the extension (.bak)

tools/testbuild.sh

@@ -244,12 +244,11 @@ function configure {
     varname=`echo $setting | cut -d'=' -f1`
     if [ ! -z "$varname" ]; then
       echo "  Disabling $varname"
-      sed -i -e "/$varname/d" $nuttx/.config
+      kconfig-tweak --file $nuttx/.config -d $varname
     fi
 
     echo "  Enabling $toolchain"
-    sed -i -e "/$toolchain/d" $nuttx/.config
-    echo "$toolchain=y" >> $nuttx/.config
+    kconfig-tweak --file $nuttx/.config -e $toolchain
 
     makefunc olddefconfig
   fi