Replaced constants with PHP enums in TDF scripts. Updated PHP version in dependency list in the README

build/scripts/TargetDescriptionFiles/AVR8/AvrFamily.php

@@ -0,0 +1,13 @@
+<?php
+namespace Bloom\BuildScripts\TargetDescriptionFiles\Avr8;
+
+enum AvrFamily: string
+{
+    case MEGA = 'MEGA';
+    case TINY = 'TINY';
+    case XMEGA = 'XMEGA';
+    case DB = 'DB';
+    case DA = 'DA';
+    case DD = 'DD';
+    case EA = 'EA';
+}