[xabuild] Improve Configuration detection (#671)

`xabuild` determines the xamarin-android installation prefix by
looking at the `bin/Debug` and `bin/Release` directories, in that
specific order. If one wants to build a project in the `Release`
configuration and both `bin/Debug` and `bin/Release` are present,
`xabuild` will use `bin/Debug` as the prefix regardless of the
configuration passed to `{ms,x}build` with `/p:Configuration=Name`.
Your project will still be built in the specified configuration but
xamarin-android assemblies/etc. will come from the `Debug` build --
probably not the desired outcome. To work around it you would need
to invoke `xabuild` as follows:

   CONFIGURATION=Release xabuild /p:Configuration=Release my.csproj

which is unnecessarily verbose. This commit makes `xabuild` slightly
smarter by making it understand the `/p:Configuration` parameter and
extracting the configuration name from it and setting `CONFIGURATION`
to this value for you. This is done *only* if `CONFIGURATION` is not
set when invoking `xabuild`

tools/scripts/xabuild

@@ -41,6 +41,21 @@ if [ -z "$MSBUILD" ] ; then
 	MSBUILD=xbuild
 fi
 
+if [ -z "$CONFIGURATION" ]; then
+	for p in "$*"; do
+		case $p in
+			/property:Configuration=*|                                                   \
+			/p:Configuration=*|                                                          \
+			-p:Configuration=*|                                                          \
+			--property:Configuration=*) CONFIGURATION="`echo $p | cut -d '=' -f 2`" ;;
+		esac
+
+		if [ -n "$CONFIGURATION" ]; then
+			break
+		fi
+	done
+fi
+
 if [[ "$prefix" == */tools/scripts ]] ; then
 	Paths_targets="$prefix/../../build-tools/scripts/Paths.targets"
 	for c in "$CONFIGURATION" Debug Release ; do